i can't imagine that will work...you will need to have a dedicated ground for each controller...assuming that the dreamcast controllers use a common ground in the first place. some game controllers do not use a common ground, so you may want to verify that first.
are you interfacing this with a pc? are there even drivers that will work with these controllers?
No its going to a dreamcast, if it was PC I would not need it I would just use the I-PAC. I found out it wont work I need seperate ground. So I have updated the plans and added in how it will look final.
I may use the one unused wire/pin for a coin input button but it would cost a lot to order just 1 button, so thats only going to happen if I find that I need one and maybe find a nice little button at radio shack.

The I-PAC goes to PC for Mame and other PC stuff, the Hacked Gamepads go to the dreamcast for MVC2

(yes lots of money & trouble for one game, without this modular setup I could have just wired directly to the I-PAC with no need for DB25 cables, soldering, switches, project boxes ect... the payoff is that I have a modular system with room to add 2 more systems easily down the road)