I think it's fine - plenty of people do this. Just don't let them out at the same time
, which I know you won't. Thin Lizzy commented recently that they sometimes compete with each other for attention when there is more than one! I think it's nice or interesting in a way for them,to be aware of another hamster around and as long as their cage/territory is sacrosanct they should be fine. When I had our robo in the living room and our Syrian in a different room, the Syrian, when roaming in the living room, was very interested in the area where the robos cage was and would have a good sniff around there (and pick up any bits of dropped food!). He seemed curious.