JACK duplex mode config

[tomtobback]

I have installed KXStudio on my laptop and want to start using Ardour to record and mix tracks. I connect my laptop with a Behringer mixer with USB interface.
When i configure JACK in Cadence, i would like to use USB for input and my system soundcard for output. However, i can't get the Duplex Mode to work. When i put the USB interface as master device/interface (top line), it works fine for input and output, but i cannot output anything on my laptop's speakers.
When i put the system soundcard as master device/interface (top line), then i cannot access the USB input.
When i put it in Duplex Mode, as would be logical, with input USB and output system soundcard, the input works, but i get no output on my laptop's speakers via JACK. Only in Audacity i can get some output when i manually switch to ALSA input and pick my system soundcard.
I am very new to this so maybe the solution is obvious, as i think this should be a common way of working?
Thanks for any advice.

[khz]

Welcome \o/

Maybe --help
http://linuxmusicians.com/viewtopic.php?p=57412#p57412