alsa config to play both through internal and external sound cards

asound.conf

pcm.!default plug:both

ctl.!default {
  type hw
  card 0
}

pcm.both {
  type route;
  slave.pcm {
      type multi;
      slaves.a.pcm "external";
      slaves.b.pcm "internal";
      slaves.a.channels 2;
      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;
  }

  ttable.0.0 1;
  ttable.1.1 1;
  ttable.0.2 1;
  ttable.1.3 1;
}

ctl.both {
  type hw;
  card 0;
}

pcm.internal {
   type dmix
   ipc_key 1024
   slave {
       pcm "hw:0"
       period_time 0
       period_size 1024
       buffer_size 16384
       rate 48000
       channels 2
    }
    bindings {
       0 0
       1 1
    }
}

pcm.external {
   type dmix
   ipc_key 2048
   slave {
       pcm "hw:1"
       period_time 0
       period_size 1024
       buffer_size 65536
       rate 48000
       channels 2
    }
    bindings {
       0 0
       1 1
    }
}

ctl.internal {
   type hw
   card 0
}

ctl.external {
   type hw
   card 1
}