Something

pcm.multi {
    type multi;
    # Soundcard 1
    slaves.a.pcm "hw:0,0";      # hardware address
    slaves.a.channels 2;        # channels
    # Soundcard 2
    slaves.b.pcm "hw:1,0";      # hardware address
    slaves.b.channels 4;        # channels
    
    bindings.0.slave a;         # should be center
    bindings.4.channel 0;
    bindings.1.slave a;         # should be subwoofer
    bindings.5.channel 1;

    bindings.2.slave b;         # should be front right
    bindings.2.channel 0;
    bindings.3.slave b;         # should be front left
    bindings.3.channel 1;
    bindings.4.slave b;         # should be rear right
    bindings.0.channel 4;
    bindings.5.slave b;         # should be rear left     
    bindings.1.channel 5;
}

ctl.multi {
    type hw;
    card 0;
}

pcm.ttable {
    type route;
    slave.pcm "multi";
    ttable.4.0 1    # channel 4; speaker 0; volume 100%                                                   
    ttable.5.1 1    # channel 5; speaker 1; volume 100%                                                   
    ttable.2.2 1    # channel 2; speaker 2; volume 100%                                                   
    ttable.3.3 1    # channel 3; speaker 3; volume 100%                                                   
    ttable.0.4 1    # channel 0; speaker 4; volume 100%                                                   
    ttable.1.5 1    # channel 1; speaker 5; volume 100%
}