There is an alternative: teach the cats to use the toilet. It’s not hard, although it does take several weeks. I’ve seen the procedure in several books and did it with our cat because I hated cleaning the catbox. The cat uses the toilet most of the time; when he doesn’t, I use the flyswatter to remind him.
Basically, you put the cat box in the bathroom (uncovered). Slowly raise it by putting more books under it every three days or so. Raise it 6 inches or so and put it on the toilet WITH A COUPLE BRICKS INSIDE TO KEEP IT FROM DUMPING. Get a kid’s toilet seat (the flat kind) and build a kitty box with duct tape and cardboard. Put in less and less kitty litter, then none. Cut a small hole, enlarge the hole over time until the kiddy seat remains, then remove the seat. With your cats’ size, you might want to leave them a step up to the toilet seat. Works best with padded toilet seats.
It’s time consuming and not perfect. Sometimes, the cat S*** really stinks. On the plus side, no more kitty litter costs and everyone thinks its a really fantastic trick.