Pain in the ass IMHO, especially as I doubt even MTW2 will be doing a very good job in representing the way the armies of old covered the whole battlefield and quite handily obstructed each other from passing until at least parts were broken and chased off. And for good reasons; the presence of a substantial enemy force in a formation's rear tended to cause mass panic right fast - after all, logically the enemy could only be there if they'd already busted your mates at some point of the line and it was already crumbling, right ?

Compared to the real thing Total War battles are highly unrealistically mobile and involve tremendous amount of small-unit maneuvering that would've been the envy of even Early Modern armies nevermind Medieval and Antique ones, so getting tangled up on this sort of stuff seems somewhat pointless.