Not all hamsters enjoy being housed together. Usually the way if u r to house them together is u need to buy them together on the same day from the same litter. If not, u may need to try & use the split cage method to introduce them to each other. This method is often used for gerbils, but it can be used for dwarf hamsters as well.
http://twinsqueaks.com/split-tank.html
You can also, once doing this, move each hamster to the others side of the cage weekly (or every few days) so they begin to recognize the scent of each other. After at least a week of this, u can begin to try & introduce them to each other. Put them on NUETRAL GROUND (like a spare cage or tank or something & watch them carefully.
The two will fight for dominace, but if the fighting gets too rough seperate them immediately! If u need to seperate them u can begin the whole thing over again (split cage etc)
Some hamsters enjoy living with others, some do not, u may have got two who simply do not like one another & who will always end up fighting..