Do you mean separating the hardware in two?
If so that isn't feasible, the Wii mode CPU is just the Wii U CPU set back to what it would be on the Wii, one core and slower. The GX block (the Hollywood) is on the same package as the rest of the Latte as well, even if technically separate hardware from the Radeon like GPU.
There is also the problem in that the Video Interface is emulated, so other parts of that hardware including the DMCU is needed to be able to output the Wii mode image. The boot process of the system may also come off as another inconvenient.
If you want proper GameCube ports for titles that support GameCube controlles, getting an original Wii model with them may be a better idea. But what could be done I guess it's try to patch games that supported it to use and accept the USB adapter, albeit if taking more work and being more complex too.