It depends. The reasoning behind using multiple drivers in headphones and speakers is the same, which is that different size drivers are more capable of reproducing certain frequency ranges. In theory a well made multi-driver headphone would probably sound better than even the best single-driver headphone, but the reason this hasn't been done is largely practical. In order to tune the headphone properly you'd either need separate amplification for each driver, or some sort of attenuator inside the headphone for the smaller driver(s). Given that we think of headphones as passive devices, I don't think this would be well received.