Multi track recording/monitoring issue

First ever post here.
 
Having dabbled since CW4 I have returned using Sonar 6 PE and put together a basic PC setup (see below) which has so far (start of the year) been perfect for me recording single instruments through the PV6 mixer.
 
However, I now need to record three tracks at once. Insert three audio tracks, assign SB Left to track 1, SB Right to track 2. This leaves me the stereo USB input from the Zoom G2.1u. I then monitor through the h/phone socket on the PV6 which allows me to hear the pre-recorded backing track and for the guitar (mono), bass (mono) and sax (mono mic'd up) to accompany.
 
Now, this is where the issue occurs - a friend has a Lexion Omega which has a stereo/mono button so he can assign the Right/Left inputs to the tracks in the software and still hear the mono instruments in both headphones when recording.
 
The only way I can separate the inputs to separate channels on my PV6 mixer is to pan hard left and right, but then you only hear bass in left ear and sax in the right ear headphones when monitoring the recording. Play back is fine, standard mono track. 
 
I like manuals and I like searching and have done both, but can't find an answer - does Sonar have "sum to mono" option for use when recording like this? I've tried monitoring directly from the (very noisy) speaker output on the SB. Or is there another routing option that I'm missing :-(
 
Don't like being defeated like this..... any help would be appreciated. Thanks.

I agree with jhughs.

If you want professional results and monitoring options then you need a professional soundcard/interface.

6PE is capable of achieving stunning results but you won't realise this by using the SB as your soundcard

Check out forum member Beagle.

He has a link in his signature to some great interfaces.