For what it’s worth, I just performed a quick test of this on one of my servers as well. I scheduled two back-to-back episodes to record, the first with one minute added to the end, the second with 1 minute prepended to the beginning.
In my test, the DVR only used a single tuner of four on the HDHomeRun Connect 4K; the resulting recordings both had the resulting overlapped content. So, even though the DVR could have chosen to use a separate tuner, it did not.
I’m running the forum preview of the new Plex Media Server builds on a Pop!_OS 20.10 Linux server.