Above and below are confusing terms in this context. Hyper-V runs on the physical CPU, and the host operating system runs on a virtual CPU provided by Hyper-V. All drivers aside from Hyper-V run on the virtual CPU. When Hyper-V starts, it virtualizes the existing context, so even if a driver starts before Hyper-V, it will end up running on the virtual CPU provided by Hyper-V once the Hyper-V driver starts.
An earlier driver could play the same trick of virtualizing the current context, and it could provide a virtual CPU to the Hyper-V driver. But then the host OS would be running nested under Hyper-V under this early driver, and performance of the host OS would be poor.
Unless Microsoft provides a mechanism for escaping from the Hyper-V virtualized environment, I don't really see how it is possible to have two hypervisors running on the physical CPU when one of those hypervisors is Hyper-V.