So I took very seriously all the comments that suggested that team teaching works best when the co-teachers trade off discussion-leading responsibilities or otherwise ensure that it's always clear who's in charge at a given moment. My co-teacher, apparently, got much the same advice from the people he'd consulted with.
And for our first few weeks, we did that. We conferred by phone the day before, talked stuff through on the drive to campus, and roughly divided up teaching responsibilities: sometimes one of us would lead virtually an entire class, sometimes the period would be more evenly divided, but while one of us was leading discussion the other would respectfully remain silent or speak only after raising a hand and being called on. Those classes all went well.
But then. . . we decided we just didn't care, or that we didn't have the time to do extensive pre-class prepping, or that we had compatible enough interests and teaching styles to just play it by ear
And it's been even better this way: we take 5-10 minutes to discuss a few things we'd like to do, and a possible order, and then we get in the classroom and just go, switching on and off as we feel like it, redirecting conversation, helping each other out, and inserting tangential observations as they seem useful.
A friend with much more co-teaching experience puts it like this: having a co-teacher is like having a roommate: no matter how much you may like a person, until you live with them, you have no idea if you can live with them.
It's not perfect. We tend to have slightly longer and slightly more awkward transitions than in a normal class, since before moving on we'll usually pause to make sure the other person doesn't still have something left to say; it's also harder to scrap or invent stuff on the fly.
And though our students are lively and engaged, we don't totally have a handle on how they experience our blended class, or how they feel about the fact that we'll get into conversations with each other in the middle of discussion, or correct each other, or interrupt to exclaim, "oh! that's so cool! I've always wondered about that!"
But, eh. We're having a good time. And I hope that our students see us learning from each other, and enjoying learning from each other--and that that makes up for the class's occasional awkwardnesses.