Quoting Dominic Sacré <dominic.sacre(a)gmx.de>de>:
More importantly, you can only use two sound cards
with JACK at the
same time if their sample clocks are synchronized.
Are you sure this applies to using different cards for recording and playback?
I assumed it mostly mattered when trying to record from multiple cards
(not that I have ever tried any of the scenarios :) .