[G-Sun]

Giving audio to video and similar a second thought.

What you'd actually prefer, I guess, is really to have a tempo-marker lock to both time and beat-position, then starting a sort of B, C, etc. alternate timelines.
This would require expanding of the concept "allow a partial measure before this marker". So that previous timeline would slide in and out of the locked one.

PiP could maybe work as well.