That's no different than using any splitter cable.  The issue isn't splitting, the issue is overloading of the PSU cable.

Uh, how do I politely say I don't think so?  I'm using splitters on every Vega I own and quite a few of my 1080 Ti's.  But let me quickly add that because power is expensive where I am, I undervolt and look for max Sols/w or max H/w.

And I've been doing this for some time so if a fluctuation was going to burn the place down, would have probably happened.

Well, my vega is on its way but I have learned a lot and try to be ready when it come
May I suggest you to check this site? http://vega.miningguides.com/
there is very detailed guide how to make it work, I will do as described here

On VEGA, only the blower fan is powered from the PCI slot.
The fan uses 28W at full rpm.
Typically, you will see around 15 - 20W during normal use.

That's a bold statement. Have you got any proof/source to back it up? Arguably there wouldn't be any need for powered risers (or additional molex+sata connectors on the Onda boards) if that were the case...

Dual I understand.  Certified, I do not.  The whole world of cabling is new to me, however.  Is there a trusted certification authority or something?   Also, I haven't been able to find any 8-pin to 2x8-pin that seem to be from reputable vendors.  Everyone seems to make 6-pin ones, but the 8-pin ones seem harder to find.  Am I missing something or are larger vendors doing this intentionally? 

Per my little research..
Credit to stackexchange question.. "Is there any limit on how many SATA power connectors i can extend" (superuser). 
Helpful link was wire gauge to current rating..  Seems 18 gauge for short runs can max out at 16A, 16 gauge at 22A, which translates to roughly 12*16 and (or) 12*22. 
As per their nifty chart, they're saying (as conservative figures) for max load for  the different wires:

Gauge	Max Load
18	192
16	264

 
I could definitely be wrong.  So eat with salt.  But since everyone's undervolting, if they're using the 18 gauge, they're probably somewhere near the top end of the allowable range.  If the cards or software fault, however, and they start pulling 250+ from the psu (does anyone know what it pulls w/o the undervolt--has anyone measured the wire?), I guess it's anyone's chance if a fire starts/they begin to melt?  For posterity and the curious, I've got vega64 lc's running on these cords. 
If anyone's got wiring like this, but is NOT undervolting, I would be curious as to how long they've been running, how many watts they've been pulling, etc.  It's hard to find solid information out there. 

Of course. I expect Your next move to be fixing another car on top of Your car so You can drive 10 persons.

Down the memory clock back to 800 maybe 900MHz and do a test. I have this same issue whit 1000 and 1100 reg mods. When miner start driver crashes the system.

Use reg mod to set voltages and overdriveNtool to set de clocks for core and mem.

Two things... 
1) You can actually do what you are looking for with a windows script...  no Devcon.exe is required:  http://vega.miningguides.com/p/auto-run-miner-without-hashmonitor.html
2) You can get Devcon.exe direct from Microsoft without an extensive installation by following the method here: (first time I posted this link bitcointalk said it was a suspicious link and took it out... not sure why but I have edited the post to break the link into two pieces... (Just remove the spaces):     https://networchestration.wordpress.com/2016/07/11/       how-to-obtain-device-console-utility-devcon-exe-without-downloading-and-installing-the-entire-windows-driver-kit-100-working-method/