[linux-audio-user] asoundrc / multi error -- "Channels count non available"

As part of my experimenting to get both of my cards playing the same stream,
I defined a configuration in asoundrc using the multi plugin, per the docs
here http://www.alsa-project.org/alsa-doc/alsa-lib/pcm_plugins.html

The complete asoundrc follows. Testing using aplay -vD XXX myfile.wav, where
XXX is the pcm name, usb, desktop and default work fine; however, multi and
ttable both give an error:

aplay: set_params:906: Channels count non available

What does this mean and how can I fix it?

Thanks
Reuben

----
pcm.usb {
        type hw
        card 1
        device 0
        channels 2
}
pcm.desktop {
        type hw
        card 0
        device 0
        channels 2
}
pcm.both {
        type multi
        slaves {
                a {
                        pcm "desktop"
                        channels 2
                }
                b {
                        pcm "usb"
                        channels 2
                }
        }
        bindings {
                0 {
                        slave a
                        channel 0
                }
                1 {
                        slave a
                        channel 1
                }
                2 {
                        slave b
                        channel 0
                }
                3 {
                        slave b
                        channel 1
                }
        }
}
pcm.ttable {
  type route
  slave.pcm "both"
  ttable.0.0 1
  ttable.1.1 1
  ttable.0.2 1
  ttable.1.3 1
}
pcm.!default {
    type plug
    slave.pcm "usb"
}