I think if the sites are healing cleanly after a week or more they may be able to move up their platforms, but check with your vet on a green light date.
The litter box is fine immediately, although if you are concerned, you can switch to a soft litter for a few days like Carefresh. Ask your vet for pain medication to take home for their 1st 24 hours or so. They should not be in pain after that, or even by the time you get them home, but it’s good to have a small amount on hand.
Watch their sites for redness and swelling, and if either becomes at all active, you can have them avoid the higher levels longer. A good way to check their procedure sites is to hold a treat above the nose so she stands up, put a hand mirror by her feet while she reaches for the treat and look at her belly to see if the area is swollen, red, yellow or oozing. It’s much easier than picking them up, and less painful. If you see any of those indications, call your vet asap.
And the vet is supposed to be available to you by phone for any and all questions you have before, during and after the procedure, so never hesitate to call for more information.