Both could stand against Shepard in Mass effect 3

Or potentially neither, maybe neither gets beat up by the robot.

I think we'd meet both on Horizon and they'd both be angry at Shepard. Only 1 would probably get hurt by the robot, probably Ashley she'd be more likely to get up close in a fight though it could also be decided by a your romance choice in 1 for maximum drama.

Whoever gets hurt would stand against Shepard in 3 but it would be the other that talks them down talking about how "we didn't trust them on Horizon but that was a mistake" so I think there'd be no chance of having them turn on you fully.

^ I really like that

Originally it was possible if you played your cards right but they wanted that mission to have impact so they decided someone has to die

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.