because you have two cats, and one bathroom it will be tough. In fact I wouldnt expect both to learn... ideally you should keep another litter box someplace else, like in the basement, following the basic rule of "one litter spot per cat".

You can buy training systems, small trays that go in the toilet and you can put litter in them.

I dont know how it happens but sometimes cats train themselves.. my sister lived by her self for a few years and rarely used her down stairs toilet, but always left the seat up.. she was sometimes puzzled that there sometimes seemed to be urine in the toilet and even thought my daughter was using it and not flushing (we visited on a weekly basis).. it wasnt until she caught her female cat using the toilet on her own that she realized what was going on... this was a cat she had since it was a kitten..

One of my own cats very rarely jumps up and pees in my sink... kinda weird..

anyhow I am just saying that because of the situation, don't expect too much.. be patient, and always have an alternate litter .