I have two kittens, that are approaching "cat" stage using one box.
Anyway, ideally each cat would have its own box, with a lid. I use the new crystal stuff which seems to work very well. No urine stench at all, and no clump/clay stench that you associate with cat litter. Now if the cat drops a deuce, there's not much you can do until it airs out.
Also, I scoop everday (except when I'm a slacker) and do a full change out every 2-3 weeks.
Lastly, use premium foods (Iams, Science Diet, etc....the expensive stuff, none of this meow-mix, friskies crap that makes fatty kitties). Basically the idea is that the food, being more nutritious, will be more digested/absorbed into the body = less stool that's less smelly.