I just hooked up a rig with a 1000w ATX powersupply running 2 1080 ti's and another 1000w server powersupply with a breakoutboard connected to it and individual cables to each card and risers, powering an additional 2 1080 ti's.

I could maybe add another card, but 1080 ti's are such beasts I don't want to get too close to capacity.

I will post a photo of what I did, but very happy with the sever PSU's and in the future, will use a small PSU for the motherboard only and a server PSU for cards.