[souffle]

This combination isn't going to work out especially with that age gap. Buying from a pet store only encourages them to get in more stock. Take one back and keep the other as a singleton if you don't have the space to possibly separate if they fight.
Two same sex hamsters will not stay together at maturity unless they are siblings and never separated or handled individually. It's not a natural situation.
Like I say return one. Robos are quite hand to handle and tame from pet stores so I'd tend to keep the young one.
Put one in a plastic storage box for now as you don't want a fight where one can be killed or hurt then you have a vet bill too!