Hi I/O,
PC´s are nutz, not to blame life!!
I´m using Sonar as well. Every soundcard is different, funny why your´s leak. Well, you can record with one mic both the guitar and your singing at the same time, and balance the volume btw these two externally .This may be better than the line in/mic in connection you´ll using, since leakage actually adds body to the recording, though there´s little manipulation after that. There´s one other way that cuts out the need for preamp and mixer which i´m using. Get a MIDI interface. These have 8 audio ins and 4 audio outs on average, more than enuff for you to do what you need to do. They are connected via an interface to the soundcard(Need to upgrade?) and they aren´t that costly anyway(got to do a bit of searching though).
Connecting 2 soundcards is a bummer, Windows often have conflicts when that happens, and pops and clicks might result. Some try it though. Using one soundcard for playback and the better one for recording.