Problem solved ! It's just that my displayport output on my 970GTX was not active.
Apple thunderbolt Display works fine with x99-UD4, ONE GTX 970 and gigabyte gc-thunderbolt 2 add-in card.
But now, I have an other problem : with two 970 GTX, the bios starts-up automatically and there is this message :
PCI OUT OF RESOURCES CONDITIONS !
ERROR : Insufficient PCI resources detected !
I have a corsair 860w power; I thought it would be enough for that config
My config : X99-UD4, i7 5930K, 32GB RAM, 2 GTX970, 1 SSD, 2 HDD