I use 1 cat litter tray between 2 piggies. I put a folded up towel on the bottom (1 bath towel cut in thirds, so 1/3 of a towel folded in half and half again), then I put hay on top. They can and do both fit in it together, but mostly one jumps in and the other stands next to it (if they both want hay at the same time). Otherwise they take turns, one at a time. Majority of poops and wees are done in there - the towel is wet after a day, and soaked after a day and a half (I try to change it every 24 hours, but in all honesty this often stretches to 36 hours). They tend to spread hay over the wet patches themselves so they're never left standing on wet towelling. They also tend to mostly wee and poo in one corner of the litter box so the hay in the rest of the box is still good to eat.