Any suggestions on other ways to stretch/mix these sources? I've been out of the multi-source game for a long time...
Stretch Markers in Reaper. More specifically, Magnetic Stretch Markers. The relevant part comes at around 1:10 here:
https://www.youtube.com/watch?v=pjKBp1b8n6o(when I saw those wavs moving around together my jaw dropped)
I did a little more digging and found this trick, to create Transient Guides automatically
https://youtu.be/GRO-ecLZqmw?t=448.
I used the transient guides to automatically locate the identical transients between the waveforms and used Magnetic Stretch Markers to line them up (snap and grid disabled) at the beginning and the end of the show. Once those were synced up perfectly, I just started scrubbing through the files looking for stuff that seemed out of alignment. I'd zoom in, look for a pair of transient guides that seemed to match up, click them to automatically create stretch markers, and I'd line them up using the Magnetic trick (zoomed all the way in to the level of plotted samples) and try again. As long as I left the playhead within the stretch-markered region I was working on, every additional sync point would add additional clarity and phase alignment on playback. If I added a new sync point and playback got wonky, I'd delete that set of markers and dive back into the transient guides, looking for a better match nearby. Then I just cleared the Transient Guides when I was done to leave it with a cleaner look.