Ah, ok, I'll test this solution.
Thanks a lot Speedstar, for your precious help.
Edit 1: Ok, it's installed, both old webdriver and Cuda, I can login, I see the desktop 1 second then it quits and asks me the login and password ...
Edit 2: I can now login with one GTX 1070 in slot 4.
Edit 3: When I re-connect the GTX 1070 Founder Edition in Slot 1, with the Katana in slot 4, locked on login. :/
Edit 4: When I disconnect the HDMI cable from the GTX 1070 Katana, no login problem.
The two cards are recognized and finally work, with no lags, Open CL and Metal work on the two GTX, calibrated monitors profiles!
I'll re test with a different cable, with DVI for example.
Edit 5: I've installed a third Nvidia GPU, a GTX 1050 single slot from Inno3D. It works but without the HDMI cable (no cables connected), with HDMI same login problem. Need to test with DVI to see if it's the driver's fault.
Edit 6: it's not a problem of connector. Tested with HDMI and DVI, of two different GPU.
With the webdriver
378.10.10.10.25.106 High Sierra 10.13.6 (17G3025) and
CUDA 387.128 , connect your monitors to your primary GPU, and use your other(s) GPU for computing, without monitors.