ADAT allows for transmission of 8 channels @44.1/48 kHz.

But due to channel multiplexing at higher sample rates you loose channels:
- 4 channels @88.2/96 kHz ("double speed")
- 2 channels @176,4/192 kHz ("quad speed")

AES allows transmission of 2 channels @44.1 - 192 kHz.

For clocking you need one clock master, best the device directly connected to the PC.
If you change the clock in your DAW project settings, then the clock changes on the UCX.
And if this device is the clock master, then the other devices ("clock slaves") learn the clock automatically from it.

The Octamic XTC has no ADAT inputs to receive clock i.e. via ADAT from the UCX, so you need to use word clock
for clocking all three devices.

Alternatively you could also use AES, but then you need to get AES breakout cables, I think easier and
straightforward is to implement word clock.

To be able to use both Octamic II @44.1/48 kHz you could think about upgrading your UCX to an 802 or even better UFX II.
But even then you loose channels on both devices at higher sample rates (>= 88.2 kHz).
The only thing what you could do it to re-cable and connect one Octamic II with 2 ADAT cables.
Then you would have 8ch @88.2/96 kHz with one Octamic II connected.

An alternative would be to switch to
- a recording interface with 2 x 2 ADAT I/O (RayDAT, ...)
- a MADI based system which has 64 channels (per MADI bus), but then you need other devices supporting MADI
- an AVB based system which has more channels and by using the new RME devices, then you need an AVB switch

Please delete your double posting and lets continue in this thresd, thanks:
https://forum.rme-audio.de/viewtopic.php?id=30054