I find this issue very interesting and i think that can help you too:


"Two Cat Are Better Than One
by
Steve Duno
...
A perfectly good way of keeping a cat happy while you are gone is to have another feline friend around, preferably right from the start. They will play together, chase each other, or even sleep together in a ball. Both will burn pent-up energy and stimulate their minds while waiting for you to come home.

If at all possible, try to acquire two cats of the same age, right from the start, to insure happy times. Siblings are the best bet, but any two kittens of the same age should do nicely. Cats of opposite gender will generally have fewer incidents of territorial or food aggression, but same-sex cats will usually learn to work out their pecking order without much fanfare.

If you already have an adult cat, you can successfully introduce a new cat into the home, provided you do so gradually. Try not to bring in another fully grown cat, as the two may fight and posture for months before finding common ground. Instead, adopt a juvenile of the opposite sex; it will be seen as less of a threat to the established, dominant cat.
When you bring the newbie home, do not give it free reign. Instead, keep it in a separate room for several days, without direct access to the established animal. Let them smell each other through the door, and communicate vocally. Be sure to provide the new cat with its own litter box, and its own food and water dishes.
After three or four days, place the new cat into a cat carrier, then let the established cat greet it through the mesh door. Don’t worry if a little hissing goes on, especially on the part of the caged cat; this is normal. Leave the new cat in the carrier for an hour or so, then release it back into its room and close the door. Repeat this two or three times each day, for two or three days.

After this, open the door and let the cats interact. Unless they display dangerous aggression (biting or severe scratching), let them work it out. If the new cat is young, odds are it will submit to the older cat’s dominance. Soon they should begin to tolerate each other nicely. Within a month or two, expect them to be buddies.
With two cats in the home, you should use two litter boxes, two food dishes, and two water dishes. Doing so will minimize the chances of food aggression or house-soiling. Also, be sure to add a few more toys and into the mix, to reduce the chance of competitive hostilities. Always pay attention to both, and try not to play favourites!

One caveat: if possible, avoid having more than two cats. Owners with two felines in the home invariably begin having aggression problems when a third pet is brought in. Seen as a “third wheel,” the third cat often gets picked on and ostracized for many months, and often never gets completely accepted by the two established “snobs.” Two can be a satisfying experience, while three often upsets the social dynamic. Though exceptions abound, try sticking with two cats for now.

Two cats are nearly as easy to care for as one. Additional food costs will be minimal, as will be the price of the new litter box, extra litter, and dishes. Beyond that, one extra visit to the veterinarian is the only other fee you will pay for having two happy, socially active felines in the home. "

I hope that this advices help!