Leave it for the next stretch goal... oh, wait! That won't work!
Yeah, I know what you think - now's the right moment, you're just about to get on your scripts, so you could plan/implement co-op along with the SP from the start... well you are right, you could.
Technically it should be possible, but you would have to consider the decision from the designer point of view i.e. you would have to imagine concrete situations the players will be finding themselves in. And there could be a lot of such situations, concerning the current scope of your game, quests, outcomes, vars and what not. Not to forget the game balance, can you balance the both modes and keep it fun at the same time? (D:OS can serve as a solid example, I agree) Additionally can/will the second player break something along the way (quest-flow, triggers, quest dialogues/story etc.) and how to avoid that? And then there are resources needed, usually more scripting and testing required, i.e. tougher bugs with even bigger brain-hungry mandibulae etc.
Take a few days for a thorough discussion, you need time for such decision.
I would personally go with a SP, and once you release a nice quality product and receive all the flowers and medals, you can plan and develop co-op on a smaller scale (one chapter or a demo) and extend it further, depending on your resources and willpower. Of course, it's just my humble opinion, you know about the analogy with 'asses' already