Well..because you are using Logic you are able to have multiple audio cards working at the same time. Logic is truly the only audio software program that allows different audio cards to work together simultaneously. I havent worked with two rosetta's but I would think you would have to daisy chain them in some way so that they both are working together to give the same clock info and not allow drifting of any nature. What cards are you going to be using? Yes a Motu 308 probably would be a good idea since it has multiple digital outs for multiple digital syncing.
Opus

Well, this is what I was thinking....

Connect the two Rosettas via word clock, connect the spdif output of each Rosetta to the spdif in of a small soundcard. This would require two soundcards. I was looking at the M-audio Dio2448--a Dio2448 has one spdif in, one spdif out--and they're inexpensive. M-audio cards seem to have a reputation for running reliably with Logic on a PC. Cost isn't a limiting factor, but I don't want to spend money I don't have to spend.

And the Motu 308--frankly, I'm afraid of trying to run a Motu piece on a PC. There are very few people who think it can be done reliably (I know, you're one of them)--Motu probably would be my interface of choice if I was convinced it'd be stable on my PC.

Anyway, I can monitor via one of the spdif outs, I have the D/A. Additional analog outs aren't a concern right now. If I need a great mix, I'm going to have to take the tracks to another studio.

I'm going to pick up the second Apogee today or tommorrow. I'll experiment with the Echo cards that I have (Mia and Layla). Unfortunately the Mia doesn't have the internal syncing capabilities that the rest of the Echo line has, unless you use up their spdif inputs to sync them together...
We'll see what happens...

Thanks for your input-