Using a toilet as a second litter box

Has anyone had luck toilet training while keeping the litter box as an option? I have just one litter box right now, and I know my cats would prefer to have more than one option. But, I really don't have a good place in my apartment to put a second box. Even if they just peed in the toilet part of the time, that would be just fine with me.

It would be great if my cats could teach themselves. One of them is very interested in the toilet when a human uses it, and I keep thinking he's not that far from figuring out that he could use it too.

They make little toilet training kits for cats.  It's called Litter Kwitter. 

You need a designated bathroom if you're going to have your cats use the toilet as a litter box., IMO.