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.

I remember something about Emo Ninja convincing either Ash or Alenko to join Cerberus but it would've contradicted their character arcs so it was cut. Were they to both survive Virmire then maybe it could have worked if you have to make the choice to save one of them anyway but the other somehow survives and gets picked up by Cerberus?... Idk, I'm just speculating here.

Okay in a alternate universe with both alive. This is how it should be. After the collector's blow up the ship both Ashley and Kaidan survive. Ashley already has issues with aliens. So the loss of her first team and the loss of the Normandy crew. She snaps leaving the alliance to be a mercenary.

At some point Cerberus attempts to recruit her. Originally she turns them down. Then some time later Shepard returns from the dead. And asks her to join the team. Since her mass effect 3 skill set is the same as Zaeed's. She could replace him altogether in mass effect 2.

Kaidan would be the one that goes to Horizon to be nosy . So he would be the one that confronts the team after fighting the bug men. If Ashley is there it throws him off more. Since he'd view them both as traitors.

In Mass effect 3 Kai Leng offs both of them? At this point i don't even care anymore.