Traktor Audio 2

Copy and paste this to ~/.asoundrc and select "djAB" as the output device to be able to select channels 3-4 for the headphone output in Mixxx:

#-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
# Native Instruments :: Audio2DJ ALSA Configuration
# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
#
#    device channels        ports
#    --------   ---             ---------
#    djA        2               12xx
#    djB        2               xx34
#
#    djAB       4               1234

#-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
# dj(a-d) :: Raw 1x1 Stereo Devices
# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
pcm.djA { type plug; slave.pcm "hw:TraktorAudio2,0,0"; }
pcm.djB { type plug; slave.pcm "hw:TraktorAudio2,0,1"; }


# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
# djAB :: Multi 2x2 Stereo Device (Ports 1-4, Channels A+B)
# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
pcm.djAB {
        type multi

        # bind hardware devices
        slaves.a.pcm djA
        slaves.a.channels 2
        slaves.b.pcm djB
        slaves.b.channels 2

        # bind channels to virtual device
        bindings.0.slave a
        bindings.0.channel 0
        bindings.1.slave a
        bindings.1.channel 1
        bindings.2.slave b
        bindings.2.channel 0
        bindings.3.slave b
        bindings.3.channel 1
}