Doesn't matter what would happen in real life, it works in the game.

In MTW Arquebusiers would destroy any friendlies in front of them, but in this musketeers fire over the heads of another unit easily. It's best not to have them right behind the other unit though, but you wouldn't want them that close anyway (Too easy for enemies to break through and engage them if you do)

Additionally firearms with ammunition as heavy as the medieval battle musket would have a significant ballistic drop, so one assumes musketeers would have to aim higher than their target... So..