I would highly suggest you don't try and run all your risers and GPU's on one rail...I must be misunderstanding the post above?
If he has 1200W - what is the problem with one rail? I haven't ever bothered with multi rails and I have no problem running 4 x 1070 and 1 x 1080 with 2 x 750W PSUs. Using dual PSUs is much more complicated for a beginner though.
You do realize some PSU's have multiple 12v rails right? A single PSU can have 2-4 12v rails. I said rails, not PSU. Please read. I am not a mind reader and have no clue what PSU model he has. I made the statement just in case. It needs to be looked at.