Thanks - but after MUCH MUCH googling, I found the answer.
It seems that I was correct in that there was an issue with it seeing my second monitor as "DFP-1"
I had to force the module to see CRT-0 and CRT-1 by adding that line in xorg.conf, Device section :
Code:
Option "ConnectedMonitor" "CRT-0, CRT-1"
Once I did that, it all works perfectly... wonder if it's a bug...