I have a theory on what might be happening that makes sense on why maybe it breaks with a second tuner, but at the same time it works otherwise and also why plex employees say the feature doesn’t exist.
I think when there’s one HDHR tuner Plex asks it to send data for the same channel and HDHR makes the smart decision to use the same tuner again and just sends data off of that. (I could confirm this if I wireshark this and see two sets of packets being sent during the overlap because HDHR is duplicating the data, not Plex.) When there’s a whole second HDHR, it has more tuners to ask and then uses a different one because it’s not trying to reuse the channel it’s already getting data from.