I use the playstation to usb adapters from raido shack.
So far, I haven't had any problems with any emulator being confused
about which controller is used for player 1, player 2, and so on.
It seems that the emulator desides which controller is used for which player
based on which usb ports the controllers are plugged into.
I've got a pair permantly attached to my main pc, and a pair that I used
on the first control panel with a pair of hacked psx pads in my mame cab.