Oh, it's just THAT SIMPLE, is it?
No offense, but you do not have any idea what you are talking about. Co-op multiplayer adds on a ton of additional challenges for how to communicate between the two players, there are player management issues, matchmaking services, and all kinds of other stuff that I don't know enough about development to think of.
Technically it shouldn't be that hard to implement co-op, now that 1vs1 is already working. Convince me if I'm mistaken (wich you haven't yet), always eager to learn.
I haven't spoken of player management. That's another story, yes. I don't want Larian to waste time on this neither if it's not gonna work. That's why it's a good idea to ask people. I'm in, and some folks on facebook as well.
Nice link by the way, never heard of RTS FPS co-op before.