Many people have more than one kitty.  5 tips to help everyone stay happy:

1)  Think up-Cats like heights.  I consider vertical space extra territory.  The more cats you have, the more vertical space you need.  Options:

                           Cat Trees
                           Book cases
                           Shelves
                           Hammocks
                           Ledges
                           Window Sills

2)  Multiple litter boxes-Open boxes are best.  Cats like to urinate in a different place then they defecate.  Have at least on more box then you have cats.  Place the boxes in different spots in the house so the dominant cat does not prevent others from reaching the boxes.  Scoop daily and use a soft unscented clumping litter. 

3)  Separate food and water dishes so cats can all eat at the same time.

4)  Your Attention-At least 10 minutes per day per cat.  Play with laser pointers, ping pong balls, etc. to keep kitties active and engaged.  A great feeding toy is the egg-cersizer.  The kitty bats the egg-cersizer to make the kibble come out of the openings.

5)  Feliway diffusers in the cats’ favorite rooms.  These synthetic feline pheromones help calm kitties and make them more comfortable.  They reduce the chance of urine spraying and aggression.