I'll be recording a live concert.  6 a Capella singers and a stereo mic on the audience.
 
Is there advantage/disadvantage to recording the stereo crowd mic on one stereo channel or two mono channels?
 
Also...
If I take an ADAT digital feed from the house mixer my ADAT sound card (Behringer FCA1616) needs to be the "master".  Behringer does not bring to mind stability.  Am I asking for trouble with the ADAT connection?  If it looses sync will the house sound suffer?  If I record both ADAT and analogue will my analogue cut out if the digital connection becomes Funky?
 
Thanks!!  This is the best place to ask questions.

I would let the soundboard be the master clock.

I'd record the crowd on a single stereo track and split it later if I need to.