We recently went from one cat to two, and they both had diarrhea at the same time. The odor absorbing litter is $9.50 for an 8 lb bag, stupid money! So we switched to sawdust pellets that they use as horse bedding. $7 for a 50 lb bag, sprinkle liberally with baking soda in the cat box.
Smells fine, cleans easy, biodegradable, and damn cheap. I only wish we'd tried it sooner.