Steven J Gold wrote:... but what would happen if I attempted to take another backup (recapture) before the Compact is restarted & completes?
Just start it and find out.
Spoiler: the second action will simply wait for the compact to finish.
QRecall uses several layers of file access coordination so that no two actions are modifying the archive at the same time. (It's possible for multiple actions to read the archive at the same time; for example, you can run a recall action simultaneously with a verify or while the archive open in a browser.)
This works no matter how you run the actions. The scheduler will hold conflicting actions that are ready to run, but even if you start an action from the command line or another (networked) computer, the actions should handshake with one another so that only one is modifying the archive at a time.