The Shulchan Aruch (274:1) brings down that on shabbas night you cut the bottom challah. The Rama adds that on Shabbas day and yom tov night you cut the top challah for kabbalistic reasons. What do you do when yom tov falls out on shabbas (sources please)?
Rav Chaim Kanievsky was asked this ( Doleh Umaskeh pg.152) exact question. He answered " Chullin 101b". Footnote 403 explains that in Chullin 101b Abaye mentions that Shabbas is fixed and Yom Kippur is determined by bais din. Therefore we learn that Shabbas is more chashuv since it is always fixed. So Rav Chaim held that the bottom challah should be used just like every shabbas. This is also the opinion of Rav Eliyashiv and Rav Nissim Karelitz.