Actually the device in control of the clock will be the one whose output you use because it will be the one that sends the clock out to be received by the other devices input. The one that sends SPDID is the one you need to use as timing master in Sonar.

Thanks for that correction, Mike!

This would be interesting, but i dont see it working?.. as iirc the motu 828 spdif is only one stereo pair. to take advantage of the total I/O the control software needs to support the additional channels.
 
FWIW I have a motu 828mkIII/firewire with a scarlet 18i20 connected via ADAT to the motu. The extra 8ch from the scarlett appear within the motu cuemix control software and all works well.