Once again I come begging for help. 
Equipment at hand is a Dell Inspiron 5559 laptop, Sonar Plat., a Behringer X1832USB mixer, and a Roland RD-300NX digital Piano. USB connection from mixer to laptop, USB MIDI connection from Piano to laptop and line out from the laptop to a mono channel input on the mixer.
And this may very well be a hardware issue at best. I am using a Behringer X1832USB mixer on stage. What I have been doing in the past to record performances is splitting my left and right channels (for other reasons I've been running all vocals panned hard right and instruments hard left) to two separate tracks in Sonar which has given me good results.
What I am trying to do now is to use a couple of softsynths being hosted by Sonar to play live as well as record the performance. All Behringer offer is the use of ASIO4ALL as a USB driver. I am trying to use the USB functions of the Behringer to record from while playing or monitoring through the speaker/headphone jack to a separate channel on the mixer in order to have better access to volume control on the softsynths, as on stage the mixer is in good reach. Using a Roland RD-300NX Dig Piano as a MIDI controller for the synths. The Synths play well and record well but I am unable to route the left and right mains of the mixer to sonar tracks. When I install the ASIO4ALL driver Sonar no longer allows me to choose Dells Realtek ASIO driver for playback. It's grayed out in the choices in preferences.  I'm certain I have explained this very confusingly, and I apologize. this feat may not be possible but if anyone would know and could help it would be you guys. Any advice here is greatly appreciated!
Thanks in advance for all you do!
Keith

Its not possible to use ASIO4ALL and your onboard Realtek soundcard at the same time.
To be quite honest, this will create havoc and driver conflicts on your set up and produce some undesirable results which I have experienced in the past.
 
You should however be able to select your devices in preferences and have them all use the ASIO4ALL driver.
im not really understanding why that wont work. 
However, I'm guessing what you might be seeing is the effects of needing to update to a better Soundcard that is capable of both audio and midi monitoring. They are not expensive and a Focusrite Scarlet will get you in the ball park for around 100 bucks.
 
The only method I see possible is using 2 computers (one for soft synths midi usb/Realtek only and use the line output from that PC and send it in to your mixing console/The Behringer.
Those mixing boards with USB Stereo line recordings capability's produce less than stellar results when it comes to monitoring and recording in multi track format. They are more often than not, used to record or monitor a stereo mix and not track by track.
I know because I have a pretty decent one (The Mackie Pro FX 16 USB). I only use it for a summing mixer or for drums (sending the Drum Mic's in to the mixer, then stereo in to my Focusrite.

Thanks Chuck,
I have been avoiding using the USB out for the same reasons, I had tried before with a smaller Berry mixer with better recording results coming straight out of a sub mix. I wasn't sure about the ASIO drivers though, thanks for clearing that up for me. The two laptop scenario is one I have entertained but was trying to eliminate some onstage clutter, also trying not to lug out an interface as well. I may just forgo the onboard synth usage and bite the bullet with Roland's onboard voices. Thanks again so much for the response and your time!
Cheers!
Keith

Thanks Chuck,
I have been avoiding using the USB out for the same reasons, I had tried before with a smaller Berry mixer with better recording results coming straight out of a sub mix. I wasn't sure about the ASIO drivers though, thanks for clearing that up for me. The two laptop scenario is one I have entertained but was trying to eliminate some onstage clutter, also trying not to lug out an interface as well. I may just forgo the onboard synth usage and bite the bullet with Roland's onboard voices. Thanks again so much for the response and your time!
Cheers!
Keith