Check out this article over at
nesplayer.com.
It discusses how to solder two NES controllers into a single port. I imagine something similar could be done for the GameCube controller, although it will probably be a little tougher due to more buttons.
However, I'm not exactly sure if this is what you want to do. In order for it to work, both controllers need to have the same button pressed at the same time in order for the system to respond. It would make complex games very difficult to play.
Note: nesplayer.com usually exceeds it bandwidth every month, so if you try to visit the page towards the later half of any given month, you'll usually get an error message. This is why I had to wait five days to respond to your query.