I dont think they would both end up on horizon, but I could see one of them becoming a spectre and the other working with Hackett maybe? obviously the way the game functions now, they have the same story path. they would need unique arcs for both of them, but I always like the idea of both of them surviving.
my first run I romanced Garrus and im on my second run now with Kaidan and im just... not into it. I feel bad because I do like him! he's just..... not garrus.