Cross sees my USB Audio device, but won't let me route to it

Hello,
I recently upgraded to the full version of Cross DJ after testing out a whole bunch of other apps, and liking it (and its helpful community) the most. The main reason for the purchase was to enable headphone cueing, without which I would not be able to use it for actual DJing.
I have a Traktor splitter cable, as well as a small USB audio interface with 1 stereo input/output. Cross works fine with the splitter cable (albeit in mono), but I would prefer to be in stereo if possible.
Unfortunately I have been unable to get it to route the headphones through the USB and the mix through the built-in audio (or vice versa); something I was able to do quite easily in other programs.
Cross sees that the USB interface is there - it appears in the Built-In Output drop-down list as a choice - but does not show it as a sound card option in the routing diagram below. I can get it to play Simple Stereo/Split Mono through the USB interface OR through the Built-In headphone jack, but not Master to one and Cue to the other, which was the whole point of buying an extra sound interface in the first place.
Please advise.
Thanks,
LEN
I recently upgraded to the full version of Cross DJ after testing out a whole bunch of other apps, and liking it (and its helpful community) the most. The main reason for the purchase was to enable headphone cueing, without which I would not be able to use it for actual DJing.
I have a Traktor splitter cable, as well as a small USB audio interface with 1 stereo input/output. Cross works fine with the splitter cable (albeit in mono), but I would prefer to be in stereo if possible.
Unfortunately I have been unable to get it to route the headphones through the USB and the mix through the built-in audio (or vice versa); something I was able to do quite easily in other programs.
Cross sees that the USB interface is there - it appears in the Built-In Output drop-down list as a choice - but does not show it as a sound card option in the routing diagram below. I can get it to play Simple Stereo/Split Mono through the USB interface OR through the Built-In headphone jack, but not Master to one and Cue to the other, which was the whole point of buying an extra sound interface in the first place.
Please advise.
Thanks,
LEN