So through my (however painful) trial and error, the first collab video will be up shortly.
The main thing was fiddling with the audio and the difficulty with videos that are not synced properly. You can get by with badly-synced video, but the audio is what takes the cake.
Also, not having a full-blown audio editing software program doesn't help. I've mainly used Audacity (free) for my own recordings and music, but having real-time linear audio editing within the video program is both a blessing and a curse. Having one is the blessing, syncing the audio with less features is the curse.
But what do I want for free? : ]
Basically the moral of the story for me was to record all levels (live audio) at a high frequency, either in actual recording or using Audacity to amplify the tracks. Then you can bring levels down to their proper levels. Just as resizing an image down is much easier than resizing an image up in size (because of resolution and sharpness), so also audio that's recorded loudly can be toned down easier without the crackling associated with audio too low and then amplified - you'd basically have too much BG noise as well.
But whatevs. It's going to be a great video, and I hope you guys love it!
P.S. Also, it's good to lay out how each video will look on the final video beforehand...this way you'll know which videos to layer on top or behind, and can position yourself correctly in each video to make a video where no "dead space" is left from overlapping videos.