multichannel sound under also with >1 cards

[Technomage-hawke]

I have a situation
where I would like to add extra output channels to my sound system.
now, I could do this by just buying a new card, but,I would rather try 
something a bit more hackish. 

the idea....

take an already existing soundcard (4 analog channels - right, left,center and 
lfe) and add a second soundcard (w/ 2 or 3  or 4 output channels) and make a 
system with 6 (5:1), 7 (6:1) or 8 (7:1) analog sound channels.

I understand this can be done with alsa (or used to be). However, I would need 
to know how to map the second sound card to access these additional sound 
channels.