Thanks for your reply!

In my current understanding, I think that's true when ES-6 is connected to ES-3, as ES-3 doesn't have ES-8's internal clock (accessed via the cable), so clock is sent via ADAT. My ES-8 is connected to the ADAT I/O of my MOTU 624, and thus gets its clock from the MOTU which would, (I think) extend to a connected ES-6. Once the clock requirement is met, it's just a question of whether ES-6's output could go to another unit's ADAT.

The complete picture is ES-8 ADAT I/O goes to MOTU 624, and speculating that ES-3 output could go to ADAT-in on a MOTU 8A, which is in-turn connected to the 624 via Ethernet.