The Talmud states that "lechem mishnah" is required on Shabbat
What is the earliest source that extended this to Yom Tov also?
L'mai Nafqa Minah?
Perhaps how one approaches 3 vs. 2 Matzot at the Seder.
For an illustration of this dynamic, see the BeHaG's position re: How many matzot at the Seder.