I have Ubuntu 14.04 and I am using Matrox graphical cards C680 to be able to connect six screens per card. All screens are different in physical sizes and resolutions.
With one card, the master screen (the one with the Ubuntu toolbar) is clearly the first physically connected to the card, from right to left.
With two cards, I can not find any pattern, except that with the screens I use, it seems to be the physically biggest which wins!
Would anybody have a clue about this situation?