I opted to do the straight to N1MM+ Spectrum window route and forgo putting WB in between. Seemed easier with one less thing to get mis-configured. I suspect that using WB either by itself or as a feed to N1MM+ may offer some capability that omitting it doesn't afford, if so, I'd like to know what I might be missing.
I tried a couple things to see if I could get 2 spectrum windows going from the RSPduo, by putting N1MM+ in SO2V mode, opening the SDR UI and selecting Dual mode then trying to open a spectrum window from the 2nd N1MM+ Entry Window, but all I succeeded in doing was locking up N1MM+ and screwing things up pretty well. I had to do some reboots, Permanently Delete this Window (not sure if that was necessary) and resetting some things the SDR UI that somehow got goofed up (it got itself set to Zero IF instead of Low IF, dropped its gain to almost zilch, and something else I've already forgotten).
Anyway, its up and working now on a single channel basis. If in the future, someone comes up with a method or updated S/W to allow two spectrum windows in N1MM+ via an SDRplay RSPduo, I'd be pleased to learn of it.
73 and see you on the waterfalls,
Victor WB0TEV / V31VP