What kind of food are you giving the kitties? Some foods are less nutritionally dense than others, and the cats have to eat more (and poop more) to get what they need. If you're feeding a grocery store food, you might want to consider switching to a 'premium' food from the pet store - you can ask your vet for a reccommendation. I noticed a big difference in my cats when I switched from cat chow to Science Diet. They poop a lot less, and it doesn't smell as bad.
Also, it's best to have at least one litterbox per cat, and you might want to switch to a type of litter marked 'low tracking' or 'less tracking'. There are even self-cleaning litterboxes on the market - if you can spend the money, you might want to look into getting one of those.