It is a little-known fact that the Conservative movement's Committee on Jewish Law and Standards (CJLS) passed a teshuva in 1963 saying that observing 1 day of yom tov is a valid option (along with other teshuvot saying that it isn't). So consider this a public service announcement for anyone out there who accepts the CJLS as an authority (and sees him/herself as a mara d'atra, or doesn't buy into the whole mara d'atra thing): you now have 1 day of yom tov as an approved option.
I can't find the actual teshuvot online anywhere (including the RA website), but one site has summaries of the conclusions. And the conclusions are the important part: the set of classical sources about the essence of yom tov sheini is very limited, and familiar to anyone who has looked into this issue, so it's unlikely that the teshuvot disagree on which sources they cite or what the sources say; their disagreement is surely about what to conclude from the sources. So the most pro-1-day teshuva, by Rabbis Philip Sigal and Abraham J. Ehrlich, says: "We declare that yom tov sheni is not a hok, a permanent enactment, but a minhag, a custom. Congregations need not feel compelled to observe other than the second day of Rosh Ha-Shanah. On the other hand, those who still desire to maintain it as an expression of personal piety, as a chumrah, might do so, vetavo aleihem berakhah, may God bless them."
I hear that Rabbis Sigal and Ehrlich went on to say: "People need not feel compelled to put more than four fringes on their tallitot. On the other hand, those who desire to wear five fringes as an expression of personal piety, as a chumrah, might do so, vetavo aleihem berakhah, may God bless them." But seriously, come on! If someone has a personal minhag to observe 2 days and is in a community that observes 1 day, I can certainly empathize with their decision to uphold their minhag, since I've been in the reverse situation many times. But I don't understand the perspective that affirms keeping one day, but then says that keeping two days is praiseworthy. Would they hold that it's even more praiseworthy to observe yom tov for three days, or to observe yom tov for the entire year and never do any work? As I alluded to with the tzitzit analogy, observing a non-yom tov day as yom tov is not meritorious; rather, it may be a violation of the biblical commandment of bal tosif. If you have a minhag that that day is yom tov and act on that minhag, then that's just fine, but then you're not displaying "personal piety", you're just being true to your understanding of the calendar. (One could make an argument that it is "personal piety" to refrain from work on the 2nd day of Pesach or Sukkot, since it's chol hamo'ed, but that argument wouldn't apply to the 8th day of Pesach, etc.)
Furthermore, it's not entirely true, on a technical level, that keeping two days of yom tov is a chumra. We'll get to that later.
My thinking about how to frame the 1-day versus 2-day options is much more in line with a 1999 CCAR teshuva that doesn't rule out observing 2 days, but advises extreme caution. It says in part: "For when we declare a second day of yom tov, we are not simply making a statement of identity, planning a creative worship experience, or arranging an experiment in spirituality. We are declaring a festival. When we say that a day is a yom tov, we mark it as holy; we transform it from ordinary time into sacred time; we make kodesh out of chol. We arrogate to ourselves the power of the ancient Sanhedrin to announce to the Jewish world--indeed, even to God--that such-and-such a date shall be a festival. And when we declare a yom tov sheni, that is, a festival day on a date that according to the Torah is not a festival at all, we create an actual festival day with all its relevant duties and restrictions."
(Read on for even stronger language, which I'm not quoting only because it applies mainly to the specific case. The question at hand was about "stretching" Shavuot to two days when the "2nd day" is on Shabbat. But the CCAR's rebuke should also be extended to those Reform and Reconstructionist congregations that generally observe 1 day of yom tov but then celebrate "Simchat Torah" on 23 Tishrei, to be cool like the cool kids. Of course, "Simchat Torah" has no fixed date and can be celebrated on any day of the year (yom tov or not), though doing it on 9 Av might be tacky. But, for the reasons stated in this teshuva, it is problematic to treat 23 Tishrei as a yom tov, in a milieu that does not otherwise recognize two days of yom tov. I don't care if everyone else is doing it. As I've said before, unity should not come at the expense of authenticity.)
The (summaries of the) other CJLS teshuvot make it clear that (at least based on these teshuvot, not looking at actual practice) the Reform movement takes the concept of yom tov far more seriously than the Conservative movement does.
The second teshuva, by Rabbi Wilfred Shuchat, calls for keeping 2 days of yom tov, and makes a slippery-slope argument: "If, however, the second day of Yom Tov were eliminated, it would not be long before the first day would fall into desuetude. We have living proof of this contention. A large and influential religious movement in Judaism has eliminated the second day of Yom Tov for the past two [sic] generations. De facto, if not de jure, the first day no longer exists as a significant factor in that movement." Rabbi Shuchat appears to believe that leaping from correlation to causation constitutes "living proof". I won't deny that both claims are true (the Reform movement has eliminated the second day of yom tov, and yom tov is not a "significant factor" in the practice of many Reform-identified Jews), but the causal link between them is without basis, and smacks of the usual intellectually lazy "If we did that, we'd be Reform" argument. Even the correlation can be knocked flat with a simple and significant counterexample: has yom tov "fall[en] into desuetude" in Israel?
The summary continues, "Rabbi Shuchat concludes by saying that he would agree to the elimination of Yom Tov Sheni if it were to come from a recognized halakhic body in the land of Israel." Don't hold your breath for any halakhic bodies in Israel to say anything one way or the other on this topic. They are, of course, already observing 1 day of yom tov, and don't generally make pronouncements about what people outside Israel should be doing. Rabbi Shuchat appears to believe that, when it comes to issues such as this, Israeli rabbis have jurisdiction in the Diaspora, but Diaspora rabbis don't have jurisdiction in the Diaspora. EV has a new comic out about this phenomenon.
The third teshuva, by Rabbi Aaron Blumenthal, says "that it would be tragic for us to initiate a program which must lead inevitably to the abandonment of the second day of the festivals. Let those who have no alternative... not feel that they are in violation of halakhah if they observe only one day. But we cannot condone the initiation of discussions about the second day in those Congregations which do have regular and meaningful services on it." The last sentence highlights a major failing of American liberal Judaism. Its emphasis is not on whether we should (as the CCAR teshuva says) declare a festival and "transform it from ordinary time into sacred time", nor on whether individuals and families should observe the positive and negative mitzvot of yom tov, but rather, on "services" in "Congregations". The implication is that regular people are insignificant sheep, with no independent motivation to pursue Jewish observance, and what really matters is what goes on in the synagogue. And of course, there is nothing preventing a congregation from having "regular and meaningful services" on 7 Sivan if it wishes (regardless of whether those services use the yom tov or the weekday liturgy), or any other day of the year. If congregational services are held up as the reason for keeping 2 days of yom tov, then this exactly is what the CCAR teshuva warns about when it says that declaring a day as yom tov isn't only about "arranging an experiment in spirituality" (even if the Conservative movement of the 1960s wouldn't have used the word "spirituality").
Neither of these CJLS teshuvot takes yom tov very seriously if they're willing to declare a day as yom tov in order to achieve short-term public policy objectives (maintaining "meaningful services" or, based on dubious evidence, preventing the apathy towards yom tov found in the Reform movement).
Furthermore, an underlying assumption in all three teshuvot is that keeping one day of yom tov is doing less, while keeping two days is more machmir (stringent). The argument in support of this assumption is self-evident, but I want to present some evidence against it (in addition to the bal tosif argument above). I've started making a list of ways in which keeping one day can actually result in practices that are more stringent than keeping two days, and you're invited to add to the list.
- The most significant one for me (albeit less technical than the ones below) is that "ששת ימים תעבד" (six days you shall work) is a positive commandment, and observing 1 day of yom tov means working on more of the six days of creation. Getting up at 5:30 AM (after staying up all night and then squeezing 2 nights' worth of sleep into a 22-hour period) to take the train back into the city from the Shavuot Retreat and go to work on 7 Sivan doesn't feel like leniency to me.
- Tefillin is the canonical example that comes up in discussions of 2-day yom tov observers visiting Israel. For those who wear tefillin, it's required on 23 Tishrei, 22 Nisan, and 7 Sivan for 1-day yom tov observers (and possibly on 16 Tishrei and 16 Nisan, but it depends on one's minhag), but not for 2-day yom tov observers.
When the first day of Sukkot or Pesach falls on Thursday, two-day yom tov observers may cook on Thursday for Shabbat (provided that they have set up an eruv tavshilin in advance), while one-day yom tov observers may not.[UPDATE: Never mind. See comments.]
- If someone is buried during chol hamo'ed, the shiv'ah clock begins ticking (for everyone) at the end of the biblical festival (i.e. the 7th day of Pesach or the 1st day of Shemini Atzeret), but mourners who observe 2 days of yom tov don't begin actual mourning until the end of the 2nd day of yom tov. Thus, 1-day yom tov observers observe an additional day of shiv'ah in this case.
- (This came up on the Hadar Shavuot Retreat last year.) At the conclusion of the 1st day of yom tov (assuming that neither day is Shabbat), 2-day yom tov observers may start ma'ariv earlier (to bring in the 2nd day of yom tov early), while 1-day yom tov observers have to wait until later. (The precise times are a matter of disagreement, but I think the relative times are accurate.)
- (One-day yom tov observers may be obligated in lulav when the 1st day of Sukkot falls on Shabbat, but that's a controversial position.)
- What else belongs on the list?