Wii U can use two GameCube controller adapters - Nintendo Everything
How to Use a Gamecube Controller on a Wii: 5 Steps (with Pictures)
HOW TO USE A GAMECUBE CONTROLLER ON WII GAMES (DOLPHIN) - bluevelvetrestaurant
How to Use a Gamecube Controller on a Wii: 5 Steps (with Pictures)
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GBros. Adapter | 8BitDo
Need help with Wii and Ocarina of Time - Wii, WiiWare & Virtual Console Forum - Page 1
TIL You can use gamecube controller in Mario Kart Wii : r/wii
GameCube Controller Adapter for Nintendo Wii U and PC USB - 4 Ports Connection Tap Converter for Multi-Player Games Black [Nintendo Wii U] - Walmart.com
Here's how to get Wii U's GameCube Controller Adapter working on Windows | VentureBeat