Nah, the Wii wasn’t even two GC glued together. It was really just an overlocked GC. When you play GC games on Wii, the hardware clocks down and the Wii becomes a gamecube.

The reason why the Wii U can do that as well, is because in adition to it’s own hardware, all the necessary Wii hardware is also on board. It’s less a Wii mode than a built in Wii.