I cannot talk about the special types of cat litter as this is above my detail knowledge about cat litter but about the number of boxes.

It may be possible that they share one box but you don't know yet.

Both my boys want a clean litterbox. Tigris will only tell you (making clear who is boss and who is slave) when it's not but Filou will use the laundry basket We have seen him using one of the freshly cleaned boxes for pee and going to the other for the rest

So if you get a cat that is relaxed about it and if Rizzo sees it in a relaxed way too, it could be possible. But if you get a spoiled young lady (or boy BTW) it couldn't work out.

Good luck however. We would love to see a brother or sister for Rizzo