Thank you John !
Ok, I read those thread... so, to summarize (and to be sure I understood everything) :
1) Premiere don't work with 2 GPU at the same time (no SLI / no 2x graphic card "alone").
2) I need to install the strongest graphic card (used in MPE) in slot 1, and the second graphic card (connected to my DVI displays) in slot 2.
3) I have to be really careful with drivers (but it's the same manufacturer, EVGA, and the same line, GT and GTX, so I don't think there will be problems)
and if I have problems, just need to desactivate the CUDA processing (in the drivers panel) of the card just used for the displays.
Can you just confirm me I'm right ?
(And, about Resolve, I saw the same slot configuration on the manual, so I hope everything will work : )
MAC PRO SLOT CONFIGURATION Optimum two GPU configuration
Slot Board Function
4 HBA or RAID card Storage interface
3 Capture card Video & audio I/O
2 Single width GPU GPU for GUI
1 Single or double width GPU GPU for image processing