Instructions
Place the hamsters' cages directly next to one another so that the hamsters will be able to see each other. Keep them in separate cages next to each other for three days, until the hamsters become accustomed to the sight of one another.
Remove the hamsters from their original cages, and place each one in the other's cage to familiarize the hamsters with each other's scent. Continue to keep them separated. Do not place one inside a cage if the other one is still inside that cage. Leave them in each other's cages overnight, and return them to their original cages in the morning. Repeat this every night for about three nights.
Place both hamsters in a large space on the floor, closed off from other places so that they can't run away and become lost. You can even let them explore an entire room, as long as you make sure there is no way for them to get out of the room. Take both of the hamsters out of their cages, and place them inside this large exercising space.
Watch them carefully, allowing them to interact with each other on the floor for about an hour. Make sure they don't fight and injure each other. If they do begin to fight, immediately separate them and put them back inside their cages. Do this every day for one week. If possible, let them interact on the floor for several one-hour sessions each day.
Put them together in the same cage after they appear to be getting along with each other. Make sure it is a large cage. Remember, the more room they have to run around and explore, the less aggressive they are likely to be towards each other.
Observe them inside the cage, especially at night. Hamsters are nocturnal animals and are more likely to fight with each other at night. If they show any signs of aggression, separate them again. If they show no signs of aggression, leave them in the same cage, but continue to monitor them closely for signs of fighting or bullying.
How to Introduce New Hamsters
Hamsters can be aggressive little things if they are not properly introduced. If hamsters don't get along, they might fight, and one might seriously wound or even kill the other one. Give your hamsters large cages with plenty of room to run around and explore. Equip the cages with tunnels for them to crawl through and wooden objects to chew on (make sure the wood is untreated). A happy hamster will be a less aggressive hamster.