No the 680 needs to be the master clock or first in the chain and everything syncs to it
It seems like the opposite might be easier. Send the AES digital out from the ULN8 to the 680 for its digital input. The 680 records those channels and uses the clock from the AES signal to clock the other 6 channels. This is how the 680 works from what I understand, though I haven't tried it out -- use a digital input on the 680 and those 2 channels and the rest of the 6 other channels are clocked to the incoming source.
I don't know about latency, but it shouldn't matter anyway -- the 680 won't start recording when the ULN8 does, you'd have to start it manually. In post, you'd have to line up the 6ch of the 680 with 8ch of the uln8, but the clocking would all be the same.
I don't know enough about the uln8, but if you can configure it to send out just 2ch of audio out the AES output, that should make things easy -- the uln8/mac combo would record those 2ch and then the 680 would record the same 2ch with the same clocking from its digital input. So the two 2ch files from each recorder would be identical, other than the different start times and thus one source having more dead air recorded up front. That should make lining up those 2 sources, and in turn lining up the remaining 6ch recorded on the 680, very easy.