Hi, thanks for the reply.
Both Pis are using TVHE version 4.2.6. The HHR Connect Duo firmware is up to date at version 20180817.
!capture.png!
The picture shows what is seen in each TVHE. As you can see I did do what you suggested and allocated one tuner to each device, and initially nothing changed, but later it was working, that is I could watch live TV on both devices at the same time. So this does work, but of course it means that only one tuner can ever be used by one device, and so I can't record and watch a different channel from the one device at the same time.
It does seem odd that if I allow both devices to use both tuners and tune just one device, I can see in the HDHR page that one of the tuners is idle, but the other Pi says there is no free adapter available. It is as if TVHE is always trying to grab all the available tuners in the adapter.