shmums wrote:thank you for all these details! I'll admit that I'm not very handy
I use a Gtx titan x 12 GO (mac pro 5.1)
Do you have a model to advise me?
Yes, this is a Mac Pro 5.1 running Sierra 10.12.6. and 2x GTX 1080 TIs with the help of the additional external PSU. It's primarily just a render node for Deadline.
The PSU we had laying around is a Corsair TX750M, which is total overkill for this, but it has a gajillion power connectors and types already on it, so I didn't spend much time looking for something smaller. Since each PCIE slot on the mother board already provides 75 watts, and the cards are rated at something like 250 watts max draw, a smaller 400 watt PSU would have been plenty.
Looking at that machine again, I see that I actually used the two six-pin power cables off the mother board, then two of the eight-pin cables from the external PSU. That worked for this particular set of basic 1080 TI cards, since they each use one six-pin and one eight-pin power cable. Depending on the manufacturer of your GPUs and whatever overclocking, fancy cooling gee-gaws, bells, whistles, and other ridiculous color changing Disco lights the've stuck on them, they might each need 2x eight-pin cables.
Here's a closeup of the little jumper I mentioned earlier, and a shot of the overall configuration. You can see I closed off the CPU/RAM compartment with some classy cardboard to keep proper airflow, but the drive/GPU bay seems to stay plenty cool open, probably because it's running a single, cool SSD instead of a bunch of hot HDs, the way it was originally intended. You can also see the optical bay is still empty, from the previous configuration where I tapped into that for powering a couple of 9xx cards.
I hope that's helpful.