Nope! I'm still trying to fix it!
Following all the advice here and mainly from Boswell. I am going to reassign the "line in" socket to be a "line out socket", run a trs 3.5 cable from the PC into the Behringer split into left/right, so I have the 2-track USB thing going on - and feed my mics via USB cable out from the mixer into the pc which is recording.
(Does that sound right or have I managed to confuse myself again?!)
This seems overcomplicated for no reason. If you use any usb unit as your audio interface, you shouldn't try to send or receive any audio signal from another way than the USB unit.
Most audio software won't cope well with having more than one unit to deal with.
Those using a mac computer can agregate audio interfaces and sound card.. Windows OS can't do it well unless the driver support all the units..
Your limitation starts with the behringer being only 2 mono channel or 1 stereo ch in & out.
If more are needed, I'd say buy an other unit that fits your need.
Most audio interfaces comes with a realtime mixer (software). It will allow you to do all the mixing required. Just choose one that has enough inputs for you need..
Focusrite scarlett are good, or audient or RME etc..
In my own opinion, cheap Behringer mixers are just crap.. (of course I just bought a 3k RME interface... lol)