You can access the other antenna ports on an RSPDuo,
but you have to run a .bat file first, which is part of the
rsp-tcp.exe program capability in the SDRUno program.
Here is a sample to turn on the Tuner2 50 Ohm antenna port:
rsp_tcp.exe -E -P1
(-P0 = PORT A, -P1 = PORT B, -P2 = HIz)
This works fine for me under rsp-tcp.exe Version 1.0.
The note indicates the "P" code for the various ports.
After activating the program, leave it running. You will see
the control data going to the RSP in a DOS box.
When set up this way, the Bias-T can also be controlled,
by the "Options (port/address)" button on the MultiPSK
Hope this helps,