Unfortunately you'll have to lose one of them no matter what. Move-outs in gamecube are uncontrollable, although you can get one of them to move back eventually if you visit enough times or if you talk to them as the character that lives in that town sometimes they'll say something along the lines of they want to move to another place, so next time you take the train can you take them with you so they can go live in another town. If you say yes, next time you board a train to another town (they won't be physically there) they'll move into that town.