Theoretically possible to run two operating systems simultaneously?

A friend of mine asked a question that got me thinking. Is it possible (theoretically, I assume it hasn't been done) to run two operating systems simultaneously on the same computer? Not in the sense of multitasking in one window, but being able to quickly switch from one to the other without restarting. Could something low-level be created that would intercept system calls (or whatever needs to be done) to make each OS think it is running alone? Or hardware itself that would accomplish this?

I'm not talking about dual booting or using virtual machines, but actually running two OS's side-by-side.

Access to the hardware must be serialized for various reasons, and it is the hypervisor's job to handle this. That is why virtualization is the only feasible mechanism for this.