Did some playing this morning. Thank you RCdiy for your video on the RX8R and redundancy, it helped. But...
I first bound the 4Xr in non-telemetry mode, by itself (nothing connected to the sbus). It did not seem to work - when unplugging it, I would get the low RSSI warning, which went away when plugged back in. I also got RX voltage reported on the Taranis. So it appeared that basic telemetry was still active. I tried rebinding several times, sometimes using different RX numbers. Is this expected?
I then bound the RX8R using telemetry on, with sbus to the X4R. Everything came up and worked. I was not in a position to do a real range check, so wrapped the RX8R is several layers of foil, and put it inside an old paint can. This dropped RSSI to about 47 on the bench. Plugging the X4R in, and RSSI immediately went up to 78. Maybe not conclusive that redundancy was working, but pretty encouraging.
Should I be concerned that telemetry does not appear to be fully disabled on the X4R?