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i hav 2 PCs each with a pci-e ATI card in each. both motherboards can take a seconds pci-e card.

1 PC has a big Vantec 500w supply.. the other only has 200w supply.

can i run 12v rail from the 500w PSU to the other computer/?? is that safe?

As long as they are powered from the same phase electric supply, then yes. You are very unlikely to use different phases on a standard domestic supply to sockets.

No, it's not safe.  Your ground potential will be different between the two machines and you will likely eventually burn one or the other out in some fashion.

Same AC source, same earth, if they're both from the same phase supply. This is no different to having a server with 2 PSUs, which they do.

If that's _really_ a concern, common the two PSUs up by strapping their chassis together to put them at the same ground potential.  But I think you're off-base.  The OP could alleviate this concern by ensuring the use of properly grounded outlets, and that both PSUs are plugged into a common source (i.e. the same power strip).

I wouldn't consider it "safe" enough to tell a stranger on the internet to do it, but I've done much worse things with electricity and both me and my house are still here.

If you do it, you should run a decent sized copper wire between the two supplies.  Personally, I'd use 14 or 16 gauge wire with properly crimped ring lugs on one of the power supply mounting screws on each case.

A ground loop is very unlikely, but why take chances?

BUT, you have to be certain that the home is properly grounded and wired.  If it's not, you could definitely have different potential between two plugs.  I've seen some funny stuff on older home wiring... like, ground wires that don't actually ground to anything.

Though I suppose if both PSUs are connected to the same outlet, you're probably fine.