If you mean using Xbox 360 controllers with Wii U Pro controllers in multiplayer,But couldn't there be multi controller support if it was modified so that the controller would always been in port 4 when plugged in?
Daku93 told me he is going to add this feature.
But you can already use 2 Pro controllers for multiplayer, I tested it.
The player numbers of your controllers might be reversed or something, though.
Thats because they are depending on the order of the controllers hardware IDs in Windows hardware list.
I asked Daku93, he told me this, and that he unfortunately cant change it.