Recording Issues

are you trying to do surround sound?  how exactly are you putting a guitar in the center speaker but not in the left and right ones?  there's no surround sound mixing capabilites in MC, you have to have sonar for that and you can't put anything in just the center speaker without surround sound mixing.

also, static is likely due to your signal path.  your output from the amp is LINE and you're putting into a MIC input on your soundcard.  that's improper gain staging and will likely cause static/noise and improper voltage levels.

I'm inclined to think gain stage and/or cumulative clipping.
If your computer has only the one input, it may be software switchable ( in Windows) to line level.
Sending a fairly hot line level signal to the mic input will almost certainly cause digital clipping (too much gain from the built-in mic preamp)
This tends to sound a lot like static.
Since it isn't noticeable until the second guitar is added, your cumulative levels may be too high.
Record loud enough that you get a good, clean signal, but don't try to make everything louder than everything else.
That doesn't work in the digital realm.
I just bumped a thread on recording levels that may be of some help.
And yes, a decent sound card/ interface can make the whole thing much easier to work with.