take two 2 channel PCM streams as different hw devices and make them a single 4 channel stream

alsa multi

pcm.4chan {
  type multi

  slaves.a.pcm "hw:1,0"
  slaves.a.channels 2
  slaves.b.pcm "hw:1,1"
  slaves.b.channels 2

  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
}