I think this is how the OSX driver works. It presents treats the adapter as one huge controller where all of the buttons and sticks of each GC pad is part of the huge controller. You could map the second players controls on your second GC pad for it to get picked up by Dolphin.
Alternatively, there is a second OSX driver that you could try:
https://github.com/mitchdzugan/osx-wiiu-gcc-adapter
Alternatively, there is a second OSX driver that you could try:
https://github.com/mitchdzugan/osx-wiiu-gcc-adapter