Syrian Hamsters are solitary animals and they should be housed alone. They are only tolerant of one another until they reach 6 - 10 weeks old and then they will begin to fight. Initially the fighting wont be bad, but as the hamsters mature the fighting will become more vicious and more frequent. Fighting could result in serious injury and even death of one or both hamsters, so they must be housed alone.
Unlike syrian hamsters, dwarf hamsters are sociable animals and they can live happily in pairs or groups. It is advisable to introduce them at an early age as older hamsters are not often keen to accept new companions. Young hamsters do not need to be from the same litter, but it is advisable to purchase hamsters of similar age and size. They are contented to live in single or mixed sex groups, but they will breed if they are in a mixed sex environment so it is not advisable to keep them in one.
When young hamsters are moved to a new home, it is not uncommon for a small amount of fighting to break out, even though they may have been living happily together in the pet shop. This is so they can establish a hierarchy between them and the fights often sound and look much worse than they really are. If they are left to settle they will usually sort themselves out but if the fighting continues they may need to be separated, but this is uncommon.
Dwarf hamsters of different species should never be housed together. In the wild their habitats do not overlap and each species has different characteristics so putting them together will be stressful and not good for their health.