Looking at the NRecord.log, I think that I may have figured out what the problem is. I'm guessing that when NextPVR has a tuner locked, tuned to a channel and streaming from that channel to a port, then changes the tuner to a different channel and directs streaming to a different port, it is presuming that the HDHR will stop streaming to the first port. But, (again still guessing at this point) the HDHR is actually streaming the newly tuned channel to BOTH ports. I just checked the HDHR's log and here's what it shows:
Since the log file doesn't ever show streaming stopping, it doesn't help a whole lot.
20170227-03:53:55 Tuner: tuner0 tuning 804 WCCO HD (auto:237MHz-1)
20170227-03:53:56 CableCARD: tuner0 804 WCCO HD (auto:237MHz-1) access = subscribed
20170227-03:53:57 Tuner: tuner0 streaming rtp to 192.168.143.10:36312
20170227-03:59:59 Tuner: tuner0 tuning 825 USA HD (auto:159MHz-9)
20170227-03:59:59 Tuner: tuner0 streaming rtp to 192.168.143.10:36313
20170227-04:00:00 CableCARD: tuner0 825 USA HD (auto:159MHz-9) access = subscribed
I just setup a new test, and during the overlapping time period, I'll check the network traffic to see if it is indeed going to both ports.