Searched for diodes (who knew?) and from what I gather this is what I need to do (please confirm or deny before I fry something useful):
Leave my grounds connected as they already are. (I have a set of controls already wired to the IPAC and one already hacked to a PSX pad-not currently sharing grounds, of course). Take the wires I've got connected to my "NO" connecters and just connect a second one to each. Connect these new wires to the appropriate thing (2nd pad or IPAC 2nd player). Now, I have to take the 2 different ground loops I have and basically attach a switch to them to either accept the IPAC current or the PSX's ground current. I then just have to flip a switch for what ground (and therefore controls) I want to have activated.
Am I close? (Yeah, I know I didn't mention soldering a 2nd psx pad, etc.) Any caveats?