I suppose there would need to be an arbiter; a system in control of which operating system is active at any one time (I presume that if simultaneous OS's were possible, the two operating systems would need to take turns and not run truly simultaenously). Also, the operating systems themselves should not be aware of any change. Perhaps the arbiter would put one OS into suspend mode to switch to the other? Lastly, there's the problem of the process of booting, whilst having another operating system running, would it be possible to switch out of that, and boot up another?!
I apologise if this all sounds ridiculous, but it's a question I've been pondering and it'd be damn exciting if it were possible!

