The ADATBridge should be connected to the Primary (MixCore) card in a ProTools system.  
The ADATBridge can only clock to "optical" when connected to the MixCore.  The option is not even available in PT's Session Setup window when connected to a secondary (MixFarm, etc.) card.
While the ADATBridge _can_ be connected to the secondary card, clocking properly to incoming optical will be unreliable and/or impossible.  Even when a common/"house" clock reference (USD, AardsyncII, etc) is used, problems will occur.  Most commonly, clicks and pops during ADAT playback, and crackling white noise when ADATs are on input (rec-enabled).

I first experienced these symptoms with a PTMixPlus sytem, and AD8000 on the MixCore, an ADATBridge on the MixFarm, 2 ADAT's and a BRC.  Even when sending word clock (from Bridge or AD8000) to the BRC, record-enabling ADATs cause PT to recieve white noise only.
I confirmed the issue w/Digidesign.