yeah that's obvious, but you don't need to completely disable one to render with another.
All it takes is just to specify for Cycles (the render engine) which device you're going to render with (either CPU or GPU)
a person with a TR 3990x and 4x2080ti will do firstly a render with CPU only, and another one with GPU only, than show us the time of each one, that's all, it's not complicated.