Well, I would NOT use the cascade function. From what I understand that is how to sync two decks without any external preamps in place. What I am suggesting is to run your 680's as 7&8 digi in. And, I'd have to say, I'm not sure why they would try to run wordclock into a SPDIF/AES input.
V3(#1) > SPDIF out to DECK #1
V3(#1) > AES out to Deck #2