I think it would require a pretty heavy rewrite of the engine to do simultaneous movement, because right now there's nothing in place to tell it what to do when two things try to move together. I also think it would give a significant advantage to the player who reacted rather than acted; watching the map for your approaching army, assassin, or whatever and just got out of the way before you arrived.

It's an interesting idea, but I don't know if the current engine could be flexed enough to make it work.