I think you could do it so that you both are siblings. You can share units if you engage with each other, when one of you goes into a fight, the person initiating it can decide to Auto-Calc or engage in battle. If you engage in battle, the other player can control either a troop or a companion, unless of course their own character is involved in the battle. The main combatant is in charge of dictating orders, while the controlling player could either be given a sergeant role such as only controlling infantry, archers or cav.
I don't think the issue is game is too easy, the game is purely a sandbox, so why limit how much they can do? If the players want to cheese big armies into engaging, then they can do it. If you really think this is an issue, then either make a radius of where another player could help. Maybe make it small enough that the armies would have to be on top of each other anyways, which any AI lord would either run from the fight or engage.
For garrisoning, you can easily let both players do it if they are part of the same faction, since this is a mechanic that other lords already do. Just don't have the players tied down to the same faction if they don't want to. One wants to lead Vlandia and the other decides they want to see bountiful horse archers? Let them do it. It's a sandbox, let them do it. No one's fun is going to be ruined cause things were too open.
For tournaments you can do the same thing in terms of radius. Both players wanna compete against each other for the prize and really make betting a thing? If both are near the town, let them both participate, just that each others fight's can't be skipped if they are different groups.