Something

 pcm.ch51dup {
        type route
        slave.pcm dmix6
        slave.channels 6
        ttable.0.0 1
        ttable.1.1 1
        ttable.0.2 1
        ttable.1.2 -1
        ttable.0.3 1
        ttable.1.3 -1
        ttable.0.4 0.5
        ttable.1.4 0.5
        ttable.0.5 0.5
        ttable.1.5 0.5
 }



pcm.!default {
        type softvol
        slave.pcm "ch51dup"
        control {
                name "Software Master"
                card 0
        }
}