So, You’re Possessed! is coauthored. We actually met as teenagers through mutual friends who were also into reading, writing, art, and gaming. It was only later we realized we had similar tastes, goals, and that our working styles just meshed. It’s possible you could find a similar group at a college, library, or other local hangout to meet more people who share similar interests. Or if you end up hitting it off with someone here on the forums, as has happened.
I would say the biggest pro is that our combined efforts have been greater than the sum of our individual contributions IMHO. For us, we think completely differently, but in a way that is complementary and helps us see another interpretation or set of scenarios that we wouldn’t on our own. And, as you rightly pointed out, you have an automatic sounding board for ideas, as well as someone to help you carry the intellectual weight of the plot, dialogue, etc., especially when you feel stuck. I guess a con would be that you might end up frustrating each other more than you make progress, but worst case is the project fizzles out and you’re free to do another one, right? I haven’t had that happen, thankfully, but I’m sure it does all the time. Even Lennon and McCartney couldn’t stand to work together forever! 
Other than that, I kinda sorta talked about this on another thread, so if you’re curious about more nitty gritty details, I’ll post that link here and avoid repeating myself. Hope something in my rambling response has helped! 