I got the CC mode receiver working. There are a few caveats I want to point out before moving on to the VC portion.
This is my original command line for starting the RTL receiver:
C:\DSDPlus.0695-1.1\FMP24.exe -i1 -o20001 -P0 -f856.1125 -g40 -rc
The -i1, -P0 and -g40 parameters generated error messages (invalid command line parameter) from FMPP, so I had to eliminate them.
This command line works with FMPP and the RSPDuo:
C:\DSDPlus.0695-1.1\FMPP.exe -o20001 -f856.1125 -rc
Note that the FMPP command line parameters listed in the latest NOTES.TXT indicate that the -i switch is still valid. The FMPP.cfg file starts with a new line for a list of RSP device numbers, which in mine is blank, so that might have caused the error.
Now on to testing voice channel tracking with the Duo...