Thanks for the suggestions.
Bob and Sam have been better about using the litterbox. I think my roommate finally got more forceful (i don't know if that's the right word) with yelling at them when they didn't use it and praising them when they did.
There are two litterboxes; I think it's called Breeze (it uses pellets, not litter).
Bob has several issues; I don't know what all of the medications are, but one is for his heart, one is for his kidneys (which causes inappetance), one is for increasing his appectite, and there's one more that I don't know what it's for. They both take medicam, and Bob also receives fluids through an IV a few times a week to keep him hydrated.