After dinner, I dumped the video out of the card, converted the MXF files, and combined them in MovieMaker. It's probably not obvious, but both audio channels are on this video. Apparently the only way to combine the multiple channels is to add the first track, then save the movie from MovieMaker as the highest-definition file it can handle, then start a new project, import the result of the first export, then drop the second audio track in and save the resultant file. This obviously loses some resolution, and it takes twice as long because you have to save the file twice.
There's some chance that we will not be able to fully wrap our heads around how to use the high-end features of the camera until we shoot the Westercon Business Meeting this year, which will be under "field conditions," so to speak.
ETA: Anyone who wants to take a crack at starting with the raw MXF files the camera produced is welcome to them. Caution: this is nearly 1 GB of raw data. The P2 camera is not particularly economical in storing data!