yes you can connect your interface to a desk as a multi-out preamp. basically you can route all the audio from the card to the onyx thru the firewire port - but you need to have paid for the firewire option with the onyx.

but with the onyx theres no need to even think about having 2 interfaces and how they work, all you need to do is group a few channels together and route everything to the desk - remember that you have 16 channels on an onyx. with a bit of clever grouping in your sequencer thats more than enough.

try this:

group 1 - kicks and bass
group 2 - hats
group 3 - synths
group 4 - percussion

etc

thats your main four groups covered and you still have 12 channells of audio free.