PCIe extender (Riser) with power from molex and including ground (wich one?)

[Quemeros]

First of all, i only have little knowing of electricity... i'm far far away of be an expert.
Almost 98% of powered risers don't include the ground conector, that wokrs perfect if you have only 1 PSU... but a lot of people use 2 PSU's (like i want to do). Then you have this problem... PSU1 power the MotherBoard (MB from here), and the PSU2 have 2, 3 or 4 GPUs... but PCIe lane have 12v and with common powered risers you will have only 12v of PSU2 but the ground is from  PSU1. Then, i think you can have a problem because recomendation for 2 PSU's that i found was: DON'T MIX THE POWER SOURCES.

Then my question are:
1. Somebody know which pins of PCIe are the ground for 12v? There are B4, B7 and A4 like he says?... To make a better risers with ground included.
2. Somebody have 2 PSU's and Risers working today, how you set it up?... It will better to plug all risers from PSU1 (primary, the same that power the MB), or in GPUs with PSU2 you plug raisers with that 2nd PSU?

Sources:
User have the same doubt but i don't found a solution: https://bitcointalk.org/index.php?topic=76121.msg2262357#msg2262357
A user that try to find what pins are the ground of PCIe but don't confirm what they found: https://bitcointalk.org/index.php?topic=76121.msg2305710#msg2305710

Examples of what i trying to do (because i don't have one...):
https://images.weserv.nl/?url=i01.i.aliimg.com/wsphoto/v0/959600473_2/PCI-E-PCI-E-Express-1x-to-1X-4x-8x-16x-Riser-Extender-Card-with-10.jpg&fnr
http://www.aliexpress.com/item/PCI-E-PCI-E-Express-1x-to-1X-4x-8x-16x-Riser-Extender-Card-with-10/959600473.html Powered riser with that special detachable thin ribbon cable you often see folded inside laptops. Won't break or tear solder joints, and dedicated solder joint for molex power

[superduh]

there's a whole issue which i don't understand regarding powering the molex connector from one psu with the graphics card pcie chargers. something about imbalance of the 12v rails between the 2 psus. i'll let the experts chime in. i know little (what you mentioned i read about too).

just get a nice big 1500w psu

[Slander]

you dont need a powered riser unless your psu is running very close to its max (which is a bad idea in general)

[caminilegroup]

When running multiple power sources you can run into the issue of overvoltage if you are not careful as most newer power supplies take constant measurements to regulate the output voltage on the different rails.

Combining grounds between the power supplies can be done with no issue.

Once that is done, take a vmm and place it on the combined ground, and test the relative 5+ (red) and 12+ (yellow) from both power supplies to ensure they are within specs.