Did you try Keyboard and Mouse for the second player? If you did, did it work?
Are you selecting the correct input device for Pad 2 in the Dolphin GCPad Configuration window? Generally you get to choose from:
DInput/#/Keyboard Mouse
DInput/#/<Name of Your Controller>
SDL/#/<Name of Your Controller>
I don't think selecting SDL has ever worked for me, but make sure to try all options.
Have you tried if both controllers respond in other games?
Edit: I just tried this out for myself on Dolphin 4.0.2 (judging by your profile, you are using this version). I used two PS3 controllers with the MotionInJoy drivers and configured Dolphin as shown in the image below and it worked in both Mario Party 5 aswell as Mario Party 7 (I've seen a thread where someone had problems with more than one controller on 7).
http://imgur.com/bmWQ6OL
Are you selecting the correct input device for Pad 2 in the Dolphin GCPad Configuration window? Generally you get to choose from:
DInput/#/Keyboard Mouse
DInput/#/<Name of Your Controller>
SDL/#/<Name of Your Controller>
I don't think selecting SDL has ever worked for me, but make sure to try all options.
Have you tried if both controllers respond in other games?
Edit: I just tried this out for myself on Dolphin 4.0.2 (judging by your profile, you are using this version). I used two PS3 controllers with the MotionInJoy drivers and configured Dolphin as shown in the image below and it worked in both Mario Party 5 aswell as Mario Party 7 (I've seen a thread where someone had problems with more than one controller on 7).
http://imgur.com/bmWQ6OL