IMO you can't make a cat or more then one, use the same box or the one you choose. They decide on their own if its ok. If you have 2 cats, put down 2 litter boxes. If they use one more then the other, or only use the one with no problems, then you can take the other box away.
That's what happened with Charlie and Ling - they both are ok to use the same box (its pretty large - which helps). I had 2 boxes down, and they never used the smaller one, so I just put it on the side