Reddit mentions of JacobsParts PCI Express Power Splitter Cable 8-pin to 2x 6+2-pin (6-pin/8-pin) 18 AWG

https://www.amazon.com/gp/product/B07611QXG4/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

Those been good to me, haven't cut them open though lol

First, I certainly wouldn't put 4 Vegas on only 750W, even if you're undervolting. It's my experience (via watt meter) that they briefly run full-speed on startup, and you could have other troubles than lack of connectors.
Ah, I see you have two PSUs. But even still, if they don't come with 2x 6+8s on the PCIe, I feel like you're running more cable than you need to. Wonder if it would handle a splitter. I'm using that to power my risers... so even if you do use two separate rails for GPU power, you could split one of them for the riser and get that headache handled.

But for what it's worth, I did not need to use the SATA power input on my H110 until I added a fifth card; I ran a single Molex rail (which on my HX1200i, has four heads per rail) to the 4-pin connectors.

It really sounds like you need to get a better PSU first. Also, if you can, I highly recommend using an M.2 SSD - <$55 for a 128GB ADATA, which I can personally vouch for. It makes the overall package so much cleaner; no need to make room for the SSD, and two fewer cables to worry about.

I use these JacobsParts PCI Express Power Splitter Cable 8-pin to 2 x 6+2-pin (6-pin/8-pin) 18 AWG https://www.amazon.com/dp/B07611QXG4/ref=cm_sw_r_cp_apa_ipYNBbJ5AE740

But only plug in 1 6 pin. Depending on the card I actually sometimes use both to power two risers.

I just noticed that that link may be a referral Link idk tho. I just copied pasted from Amazon. Sorry of it is

How about these?

https://www.amazon.com/gp/product/B07611QXG4/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

>Is the 8-pin connector on the psu side different somehow that it doesn't have the same 150 watt restriction?

Yes.

Right side is PCIe connectors, regulated by PCI-SIG specification. Left side are various proprietary PSU connectors, connected to the PSU's 12V/5V/3.3V rails, not held to PCIe spec.

>Or are you saying that the daisy-chained cables are really set up like the right side of the diagram? If so then do they even make cables like the left side that they're implying are arranged in serial?

You're thinking too deep about it at this point. Right side just shows basic splitters.

edit: splitter like this (https://www.amazon.com/gp/aw/d/B07611QXG4)

splitter.

Aren't 8 pins rated at 150 watts? If so, then having more than two 75 watt draws on one 8 pin cable would still overdraw.

Unless I'm missing something. Which is very possible.

Hold on, I'll find the splitters you need.

EDIT: Here you go:

https://www.amazon.com/JacobsParts-Express-Power-Splitter-Cable/dp/B07611QXG4/ref=sr_1_8?ie=UTF8&qid=1518721229&sr=8-8&keywords=pcie+8+pin+to+dual+8+6%2B2

EDIT 2: You can use the 8 pin EPS 12v CPU lead as well:

https://www.amazon.com/gp/product/B078L9372P/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1