It would be cool if it was done properly, but that's the key phrase right there. I'd be up for it on the condition that it didn't detract any from the single player element, since that's what I'm there for in the first place. Honestly though, it sounds unworkable. It will take too much time and eat up too many resources to vet each and every possible scenario for both one- and two-player capabilities. Taking the Two Worlds II route and having a seperate landmass for single player and multiplayer, in my eyes, goes against everything the franchise stands for.
Then there's the problem of how the second player is going to act once ingame, hypothetically assuming the feature is actually implemented; I certainly don't want some random character plucked from cyberspace to be dropped into the game I'm hosting, since the first thing they'll probably do is head straight to my house and steal all my loot. Even assuming we don't squabble over the spoils of war or constantly rob and pickpocket each other, what if we don't choose to roleplay the same way, since that's at the heart of your co-op idea? What if my partner is playing a virginal Imperial handmaiden specialising speechcraft and healing spells bought from the chapels she frequents all around the province, while I'm a schizophrenic Dark Elf stalking the night with a dagger in my teeth and a lockpick in my boot, looking for prey to kill in the name of Sithis? That's going to be difficult for both of us, even discounting technical limitations and intense griefing.
tl;dr: It's a nice idea with a lot of big problems, so it should probably stay just a nice idea.
