I grow mine in a five gallon bucket. I've found that as long as I cut it down every fall, it doesn't get thick enough to kill itself out. It survives our winters outside just fine being in a container. I do make sure it is protected from the north and west winds.

I don't know if one, mint or lemonbalm, would be aggresive enough to crowd the other out or not. I will look into that and let you know!