The co-op singleplayer would be hard to implement from a balance and time passage stand point, I think is Taleworld's argument? I think a simple solution is to keep the game going in real time when one player is in a battle, so that the other player can come in and join the fight, move around the map, or take advantage and siege a castle while the opponent is occupied in the field. However, when both players are in a battle or a scene, time stops and the AI stops making moves on the map until one player exits the fight or scene. Not hard Taleworlds.
Honestly, how hard is it to implement? Unless they kept their horribly near sighted, hard coded crap in awful places like in Warband. I mean who cares if it is unbalanced or a bit buggy, i'd rather have the option to have some great moments with my friend rather than just nothing at all. Seriously, Taleworlds has had like 4 years now, and they will probably have been in development for 5-6 years when it finally comes out. Doesn't seem like a huge project. Hopefully a modder can make it possible with Bannerlord since that's what Taleworlds is banking on that modders will fix the flaws they left in the game.