I always thought wife and kids
were the naughty stuff.
Thanks for the input. Might look into VAC a bit more.
Interestingly enough people claim that under XP it was no problem to have parallel audio streams through the different interfaces. I guess the changes in the audio implementation in Vista have removed this

But it's really an unnecessary restriction.
I still had VAC installed, so I had a play, had everything up and running in 60 seconds (I have both onboard speakers and a usb headset)
Install VAC to get a "Virtual Audio Cable" device, which you can set as your default sound card in the windows sound control panel. Then run two copies of Audio Repeater, one with input of the VAC, and output of your first soundcard, and the other with input of the VAC and output of the second soundcard.
Win7 will let you control the output volume levels independently on each sound card.