Alright, here's what you want to do. Go either to
www.gamestop.com or go out to a local video game store, and pick up either A:Pelican or Gamestop digital psx controllers, or B:MadCatz digital psx controllers, they should range from 3-5 dollars, get two of those. They are extremely easy to hack, and if you break one, you can just go buy another.
Then either go on Ebay.com, they're harder to find there, or search the web for 2 psx to dc convertors, the kind that's a solid looking box with a black input that sticks out for the psx controller is the best kind. Now wire up the psx pads to the terminal strips the I-pac is connected to, or whatever you want as long as they're connected to the wires that go to the buttons, and fire it up, just don't have em both on at the same time. Note: if you're worried about having one draw too much power from the other incase someone accidently is stupid in your house and turns em both on, then for safety you can install a switch, a DPDT will do, and just wire the seperate grounds onto either side, so like
DPDT Switch
( )
\\
___<=======>___
{___________________}
II II II
[ I-Pac 2 Gnds ]:[ To buttons ]:[ Psx 2 Gnds ]
*Always wanted to draw some ASCII art here, but there,
there's how the switch would look and what you'd wire it up like.
-Good Luck
-CthulhuLuke