yes, but which ground? it always takes the path of least resistance... hence in my example 1 - grounds back to the supplying psu, 2- grounds into psu 1, which inturn would dump back into psu 2, or psu 1 will continually draw current from psu 2, getting greater and greater until something goes pop and the magic smoke escapes.

also it doesnt have to ground 0v (7volting fans is an example of 12v -> 5v) the thing is, without better knowledge of switching power supplies, or how pci-e connectors are attached to the motherboard I am just guessing.

remember I am not talking about powered risers in this example, but the things that connect a molex directly to the pci slot... like the EVGA powerboost (i mistakenly referred to is as an XFX product) There is an image of it installed halfway down along with some evidence it stabilises and boosts the 12v rail.  I can see how this would work if you use just one psu.  My question is what happens if one psu is powering this and another the rest of the motherboard.

unless I am overthinking it... I have some spare mb's and psus so i guess I will find out soon