Having exactly the same issue. Prefer to scratch on the left side, and it's not posssible to get the standard audio output to the right side. I'm on windows btw.
My workarround is to use the onboard soundcard, hook that up to the right line-in and setting the windows sounds to the onboard soundcard. This way I'm able to scratch to a looper.
Btw it's a shame that something like this has been overseen in developement, especially on a scratch mixer.
I had a NI-Tractor soundcard before, it also offers 4 in- and outputs, and you can use all those for the windows sounds, loopers, Itunes or whatever aswell as in tractor to controll 4 decks.