The “Duplex Browser” will only let you LOOK at a single device at a time, but you can RUN any number of devices in the background.
Open the browser, hit “Run”. Then select you other device, hit “Run” (I guess you’re already doing this).
If you want both to start to start (run) when Renoise starts, enable “Autostart configuration”
If you want the UI to show when Renoise starts, go to the Duplex menu and enable “Display on startup”
(2) You can remove any mappings you don’t like and they will be freely available for standard MIDI mappings.
For an introduction to editing Duplex config’s, look to this introducton, or the Duplex manual.