# /etc/modprobe.d/snd_usb_audio.conf
options snd_usb_audio vid=<vid1>,<vid2> pid=<pid1>,<pid2> index=<index1>,<index2> enable=1,1
example:
# 5: M-Audio Fast Track Pro
# 6: Focusrite Scarlett 6i6
options snd_usb_audio vid=0x763,0x1235 pid=0x2012,0x8012 index=5,6 enable=1,1
See also: http://alsa.opensrc.org/MultipleCards