Quick update, it appears that having the spdif signals synced allows the decks to be synced. More later.............
Ok, really just trying to mark the new thread, but....
Yes, I would imagine if you sent the same spdif signal to two 680's, they all then would be synced using the same clock as the spdif source. So that would effectively give you 14 channels (with the 2 digi channels on each deck repeated).
On my SD 722, you could connect it to a digi spdif input, but instead instruct it via the menu to actually record the analog inputs. it would record the analog inputs, but use the spdif signal for clocking. Wonder if there is a way to do this on the 680, or if not if it could be implemented with a future firmware?