This may not even be possible and have too much hard-coding involved, but is it possible to have someone mod the game so that in the MP Campaign Mode, you can "gift" units to the other player on the campaign map? The reason I ask this is because since you can't have the other player spectate a battle against the AI (you have to fight each other or auto-resolve), and you can't have the other player share your units on the battlefield, maybe a more simple solution is if I know a battle is coming up (like I'm getting ready to move onto an AI army with my own stack), I give some of my units to my friend beforehand, that way he can at least participate in the battle without fighting against me.

This may be beyond the scope of what a mod can do, but it doesn't hurt to ask. And I don't really care exactly how it's done, bottom line is I just want the option of either having my friend spectate the battle, or he is able to participate in fighting alongside me against the AI, whether that's me giving him units beforehand, during the pre-battle window, or whatever the case may be.