Having a pair or group of hamsters is lot's of fun, however there is always a chance of them fighting. My first pair of dwarfs ended up needing to be separated due to fighting but through that experience I learned some basic guidelines for housing multiple hamsters and so here there are.
First off, Syrians must not be kept together as they are solitary creatures by nature and will fight to injure or even kill each other. This could lead to very expensive vet bills if not then the death of your hamster(s). Even if your syrians are not fighting right away, living together is very stressful. Stress lowers the immune system making the susceptible to illness.
Dwarf hamsters, on the other hand may live together. There are four species of dwarf hamsters that are commonly kept as pets. There are Russian Campbell (RC) Dwarfs, Winter White (WW) Dwarfs, Roborovski (Robo) Dwarfs and Chinese Dwarf hamsters. Each breed of hamster needs to be kept in a group of its own kind, Campbell with Campbell for example.
Some species do better in groups than others. Chinese hamsters (females in particular) have been noted to fight more often and may be the least compatible for group housing. WWs seem to do well in same-sex pairs. RCs and Robos tend to be most social.
Some tips for keeping multiple hamsters are:

Make sure you have a large setup for your new hamsters. If you don't have a large enough setup then your dwarfs could fight for space. The accepted minimum floor space is an area of 360 square inches. It is not recommended to have multiple cages for a pair/group of dwarfs as all too often the hamsters pick one cage and become territorial over it. It is best to have one large cage.

A large food dish. Most dwarfs love to be able to sit in their food dish. But if you have a food dish that can't hold a bunch of dwarfs then you could be asking for a fight. Food dish's that would allow all of the hamsters to sit in at the same time are best.

Sometimes one or more of the dwarfs will be greedy. So the other(s) might not get enough food and then get angry. So if you find your dwarfs are not getting the right amount of food the answer is not another food dish. Why? Because now Piggy gets twice as much food. I would suggest separate feeding times. Feed one and then take the other out. Then switch turns. But usually having a large food dish can help prevent this.

The next thing you need to consider are the wheels. Never get the wheels that are actually made for dwarfs. Some are too small in the first place which can hurt their spines. Besides that not all the dwarfs will be able to fit on it at once. A syrian sized wheel or bigger is your best bet. You also need a wheel per hamster. I usually use the medium (6.5") Silent Spinner Wheel.

Sometimes water bottles can be a source for a fight to start. Dwarfs may fight over getting a drink. If they seem to be squabbling when drinking you will need to get another water bottle.

You also will need multiple beds. Dwarfs may not want to sleep together all the time. So it is important that you have more then one bed for them in their cage in case one is chased out of the bed. I try to have one for each hamster plus an extra. So if you have a pair then you need three beds.

The gender factor should also be considered. It is important that you have dwarfs of the same sex. Breeding is a lot of hard work and takes a long time to learn. So make sure you either have all girls or all boys.

When it comes to cleaning cages make sure you leave some bedding left over from the previous week which has their scent on it. This is because if you use all new bedding their scents will no longer be in the cage which may encourage fighting and upset the 'pecking order' your hamsters have established.
If fighting occurs and there is blood/injuries you must separate them immediately. This means you must have extra cages ready in case of a fight. Basically the rule is no blood no foul. Keep in mind that sometimes hamsters argue and what can look pretty awful is just a disagreement. Even human siblings don't always get along.
An Ideal Cage Setup, two RC dwarfs living in here (20 gallon long, 12"W by 30"L):
This post has been edited by Christmas_hamster: 10 July 2010 - 04:13 AM