If it works as a dual mono set-up when you use the same config files with the two units swapped, that sounds suspicious: maybe the AES/EBU input of one of them isn't working properly. Might be worth having a chat to Devialet and/or your dealer.
Roon (Mac Mini), Wilson Benesch Full Circle, Expert 1000 Pro CI, Kaiser Chiara
Warwickshire, UK