Lots of people are wanting to keep multiple hamsters. So here is a guide to help you.
First off Syrians must not be kept together as they are very solitary 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).
Dwarf hamsters on the other hand may live together. There are four breeds of dwarf hamsters that are commonly kept as pets. There are Russian Campbell Dwarfs, Winter White Dwarfs, Roborovskii 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) tend to fight more often and may be the least compatable for group housing. Winter Whites seem to do well in same-sex pairs. RCDs and Robo's 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.

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 seperate feeding times. Feed one and then take the other out. Then switch turns. But usually having a large food dish can help this.

Wheels. Never get the wheels that are actually made for dwarfs. Some are to small in the first place and well all the dwarfs can't fit on it at once. A syrian sized wheel or bigger is your best bet. Having a wheel for each hamster is a good idea as many hams like to run on their own.

Water Bottles. Sometimes dwarfs will fight over getting a drink. So then you need to get another water bottle.

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 incase one is chased out of its bed.

Gender. It is important that you have dwarfs of the same sex. Breeding is alot of hard work and takes a long time to learn. So make sure you either have all girls or all boys.

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 seperate them immediately. This means you must have extra cages ready incase of a fight. Basically the rule is no blood no foul.
Ideal Cage Setup, two dwarfs living in here:
This post has been edited by Christmas_hamster: Mar 10 2008, 11:47 PM