I get what you saying - and if you say you like this idea, even though you don't like most mods, I'm happy!
We certainly could brainstorm about the spawn conditions. maybe some other method would be better. - maybe a poll could be good, but what should be the options?
but spawn conditions aside, I think it would be best to leave it to the player if they want to have both NPCs, or just one. you could disable one (if you have sheat sheets mod for example, if you not want that girl to move in to that world) or just ignore that she moved in and never buy anything from her. What I mean, I think it would be better to leave it to the player to decide if want to have both NPCs or just one of them (or none, but then no point of this mod XD)
You could always make a options config file in the mod, that enables or disables said features. SOme mods have that and it works nicely.
Or maybe the NPCs could sell a special item that, when held, will make the current evil biome girl spawn. So that's both of them. This item could be a furniture piece or a banner
Crim - Defeat the Eater of Worlds AND/OR buy the special item "Down with the Crimson from Corru
Corru - Defeat the Brain of Cthulhu AND/OR buy the special item "Down with the Corruption" from Crim