I have two dwarf hamsters and they currently have two different cages.
I moved them to the bigger cage for a while and they were in it about 5 months until they started fighting. One of the hamsters loves to bite the other ones tail!
I noticed cuts on the other hamster so I was thinking about separating them into different cages.
However, I thought i'd let them have a chance to sort themselves in the other cage (which is smaller) and they have stopped fighting. They still play fight at certain times but they aren't hurting each other.
I thought I would post this if anyone else was having similar problems as it may help you to sort your problem instead of having two very lonely hammies!