I own both a MK1 and a MK2 DR-680, but have not tried to chain them. If I get a chance I'll give it a go and report back, but it may take a few weeks. You might check the DR-680 threads (there is both a MK1 thread and a MK2 thread here at TS), I think others have tried this and reported back, but I can't recall what the outcome was.
I've not had problems getting either of these to play nicely over a SPDIF connection with other gear. That includes passing digital signal to and from an OCM Edirol R44, digital in from a V3, and digital out to a home theater receiver.
Typically when I needed more than 8 recording channels (8 is achievable with 680+V3), I'd run a SPDIF cable from the R44 to the DR-680. 680's clock would lock to the SPDIF signal from the R44. Transport controls needed to be operated independently, and the files aligned later, but everything would be in clock sync. I could record 10 channels that way- 4 on the R44, 6 on the 680, plus an optional copy of two channels (or a basic mix of the 4) from the R44 on the stereo channel of the 680 as long as I had sufficient card space available, which made file alignment easy.