Comment 2 for bug 804639

I posted a dup at https://bugs.launchpad.net/mixxx/+bug/1268258 - now closed. Same problem, slightly different solution (I don't think it should be a UI option, but in the preferences):

As described here: http://www.digitaldjtips.com/2011/07/djs-guide-to-splitter-cables/ , Stereo to mono splitter cables are useful in situations where you don't have access to a soundcard with multiple outputs, e.g. for practice situations. Also, this is useful in live situations with mono audio (apparently many clubs do this to prevent phasing problems anyway. It would be really useful feature to have in Mixxx.

Basically, in Options > Preferences > Sound Hardware > Output, the Master and Headphone tracks would need a "Mix to channel 1" and "Mix to channel 2" option for each sound device.