Just go ahead, connect 1 cable with pigtail, 1 x 8 pin from the other cable, 75 watts from the pci-e motherboard slot. I am running my GPU like this with no issues at all.
https://community.thermaltake.com/index.php?/topic/349713-help-with-cables-for-toughpower-gf3-1000w/
where they are saying...