I had a newborn stash of pre folds, kissluvs, and covers, but once out of the newborn stage, I had about 15 diapers per kid for our regular stash with a few "didn't do laundry, need something" there to use. I found I was just doing laundry way more, so I didn't need a ton of diapers. Just enough to last 2 to 3 days per kid.
I never got to the point where I had two in the same size at the same size, but I have 24 prefolds in each size. If both were in the same size I probably would like at least 36. I like cloth diapering but I don't like doing laundry all the time!
Thankfully DD pt'd herself before DS arrived but this time I won't be so fortunate. DS will only be 16m when the new one arrives. I won't do diaper laundry every day so I need about 15-20 per kid.
Update, we're going to try to teach my daughter how to use the potty. We have ECed off and on her whole life so it's not completely foreign to her. If we get closer to my due date and she isn't on the potty full time we're going to increase and simplify our diapers to 30 pockets.
Re: Cloth diapering two
You could try PT'ing LO1.
DS1 was PT'd by the time that DS2 came along (19months).
We had 12 BG AIO smalls and 36 BG 3.0s to cover DS1 overnight and DS2 all day.
Good luck!!