Our 2-bedroom apts each have a Queen size bed and a Full/Double size bed. For the Queen, we put a Queen fitted sheet and a Queen flat sheet. For the Full/Double, we use two flat Queen sheets. So, we only buy Queen flats and Queen fitteds and it is probably one of the best decisions we made when we opened up, it’s so much simpler.
It really only works well to use two flat sheets if they are a size larger than the bed. If you have only Queen beds, you could pull it off by buying only King sheets (like nice hotels). I definitely don’t recommend using a Queen flat sheet as the bottom sheet on a Queen bed, it’ll pull out and be annoying and gross (like cheap hotels). The only thing is that Kings cost significantly more, so it may or may not be worth it to you, depending on your situation.
I was so worried about that issue when we made the choice to use the Queen flats for the bottom sheets on the Full/Double! I absolutely can’t stand a sheet coming out like that and I also untuck the top sheet to sleep. I bought these “sheet suspenders” in case it was a problem, but after 300+ guests, the bottom sheet has never been untucked even when the top sheet was and even when the rest of the bed looked like it had a really good time. The trick seems to be to go up a size.