You did not mention what kind of hamsters you have, but I will assume they are one of the two popular species kept as pets – the Syrian or Russian Dwarf. Syrian hamsters do indeed need to be kept individuals since they are highly territorial and come together in the wild only to breed. They may tolerate other hamsters when they are still young, but fighting that can often result in death will begin at around 8 to 10 weeks. If your hamsters are Syrian they must therefore be separated into two separate cages. However, Russian Dwarf hamsters are highly sociable and actually prefer company. That said, they usually require to know their cage mate from a young age, so do not consider adding any more hamsters.