Is it possible to dual boot two OS's at the same time?

Most, if not all, mainstream operating systems expect to have exclusive acccess to the host computer's low-level functionality (eg: hardware, ports etc.) and do not have any concept of sharing with anything else - it just won't work - so to try and run two OSs simultaneously on the same system requires some form of transparent arbitrator to shield the operating systems from each other - in other words, a virtualization application.

The closest thing you could do... is make use of hibernation. Unfortunately, the operating systems are alien enough to each other that there is no way for both OS'es to be running at the same time without implementing virtualization. Honestly, virtualization isn't as bad as you think anymore... especially when you are using a virtualization platform that supports the "VT" extensions. The guest OSes get the ability to communicate directly with hardware, and have much better memory mapping & such. I've actually setup a guest OS installed directly on a SSD... and it ran faster than the host OS.

Here's the simple, short reason the answer is 'no': What software would control your hardware?

If OS 1 does it, then you are not running OS 2. If OS 2 does it, then you are not running OS 1. If they both do it, then then some other software would have to mediate their shared access to the physical hardware, and that's virtualization (which you ruled out).

Things that come to mind are :
- Memory management ( ram and cache )
- Process management (process threads running )

So you would probably need a man in the middle juggling memory usage and processes between the two OSs .

That would be another os/firmware.
So if somebody has implemented it os will implemented it , you might end up with 3 instead of 2 Oss running simultaneously

EDIT-
Adding what linker3000 wrote : managing of ports

This 'man in the middle ' will essentially become your host OS and the other OSs virtualized , leading you back to what you already knew was a solution