Im quite sure its safe to connect them up how I suggest. AS LONG AS THEIR CASES ARE JOINED.
All the odd behaviour you see is when one is ON and the other is OFF.

What will happen with my design is the main PSU gets pulled on by the MB
(via the green cable), and because the secondary is joined to the same green,
it now gets pulled on at the same time. This is Zero volts when ON, 5v when OFF.

Joining the blacks is to ensure the Zero-Volt rail is the same level in both.

And joining the cases is to ensure the secondary one is properly earthed for safety.


The reason your PSU shut down for a while is normal (a 'better' quality PSU).
It realized something was 'wrong' and put itself into safe-mode for protection.
As soon as the power started flowing INTO it from the GPU-power cable, it went into safemode.
BUT, power was still flowing into it from the other PSU, and that put power to the
12V rail (powering the case fan), and the 5v rail(powering who knows what).
But the MB still saw no 3.3v, 2.5v, -5v, -12v rails so it stayed powered down.
But the power was still 'passing through' it. I dunno how safely though ...

As for the PC not starting when the external is off, this MAY be because it is
OFF, it is dragging the 5v and 12V too low for the main-PSU to consider it safe to start.

'Power' or 'drain' is definately passing through the GPU and causing issues
when either PSU is off. No doubts at all. Its getting through either from the
GPU power-cable, or through the PCI-X slot of the MB.


Regardless of all that, have a dig on the WWW. There are new MEGA_HIGHend PC's
out and about that come standard with two PSU's, specifically for big/dual GPU's.
You may be able to find specs on what cables THEY use to join them together.
They sure-as-hell wouldnt "manually" splice cables together, and the MB's
dont have any special plug-and-socket configurations to allow it.
Give it a try...

I may look later, depending on how far away the sandman is... he's sneaky...