I think I found a way of monitoring even with a single output soundcard (I have a Realtek HD Audio on my notebook).
It's a bit complex but seems doing the job.
I've done it in Windows, but I believe the same results are achievable in Linux and maybe even in OSX.
I downloaded and installed ASIO4ALL (http://www.asio4all.com
) and Jack (http://jackaudio.org/
) and configured Jack as described here: http://jackaudio.org/jack_on_windows
Then I started Jack Server (Jack PortAudio), Jack Control and Mixxx.
In Mixxx "Options" -> "Preferences" I choose "Sound API: ASIO" and matched sample rate and latency to Jack Control "Setup..." configuration.
Then I choose for Master "JackRouter - Channels 1-2" and Headphones "JackRouter - Channels 3-4".
I went to Jack Control and open the "Connect" window. Here I connected Mixxx "out1" and "out2" to System "playback1" and "out3" and "out4" to "playback2".
Now I hear Master Out on the left channel and Headphones Out on the right channel
Hope it's useful
(I'm new here so I hope I haven't done anything wrong ^^")