Just to add that although word clock is not necessary in this instance, if both your interfaces have it then it would certainly do no harm to use it. If you have a really good A/D with low jitter, then use that as the master.
I use an older computer running Linux (and occasionally XP with the STeem Atari ST emulator) with an E-MU 1010 PCI card synced via ADAT into an RME RayDAT and it works a treat. I make the RME the master in this case as it has the better clock, but the sync happens via ADAT.