Normally don't make videos like these, but I couldn't find ANY solution to this issue anywhere. I really wanted to do some playthrough's of some Gamecube games eventually, but I also needed to have a second controller working. After some fiddling around and just guessing what would work, I found the solution!
IT MAY OR MAY NOT WORK FOR YOU BUT AT LEAST GIVE THIS A TRY!
*Written guide*
1. CONFIGURE BOTH CONTROLLERS YOU WISH TO USE SEPERATELY AND SET THEM ACCORDINGLY (I don't know about setting up more than 2, but if this works when setting up the 3rd and 4th controllers then just apply these steps with all 4 controllers).
2. RESTART DOLPHIN (just in case)
3. REMEMBER TO KEEP ONE CONTROLLER PLUGGED IN BEFORE GOING INTO THE GCPAD SETTINGS (having both plugged in when booting up the software is OK)
4. KEEP THE SETTINGS OPEN AND DOUBLE CHECK TO SEE IF THE PADS ARE SET TO THE PROPER CONTROLLERS
5. PLUG IN THE OTHER CONTROLLER AND WAIT A FEW SECONDS, TEST TO MAKE SURE BOTH WORK AND FUNCTION IN THE SETTINGS
6. CLICK 'OK' AND OPEN THE SETTINGS AGAIN TO MAKE SURE THEY'RE FUNCTIONING (also just in case the newly plugged in controller doesn't respond in previous step).
7. OPEN ANY GAME THAT HAS MULTIPLAYER AND TEST
8. IF IT WORKS, YOU DID IT!
9. DO THIS EVERY TIME WHEN YOU BOOT UP DOLPHIN AND YOU CAN PLAY MULTIPLAYER WITH YOUR FRIENDS/FAMILY WITHOUT ISSUES!
Hope this guide helped!
•••Check out my other stuff here!•••
►Tumblr: / halomillennium
►Facebook: / halomillenniumyt
►Twitch: / hmillennium
►Twitter: / halomillennium
►Google+: http://tinyurl.com/halomillennium-goo... #HaloMillennium