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.

You seem to be dismissing Hyper-V and Xen (so called "bare-metal" hypervisors) even though they're closest to what you seem to be asking for. Yes, it's virtualization, but not in the same way that VirtualBox works.

Imagine something like VirtualBox was its own operating system, so you could install a tiny VirtualBox OS, and then run Windows and Ubuntu side by side as virtual machines on top of the VirtualBox OS. Well, that's essentially what Hyper-V or Xen do. It's just a thin virtualization layer between the hardware and the guest operating systems, and it's as close as you can get to directly running them side by side with existing hardware and existing operating systems.

You don't need a separate server for either of these, you'd just install the hypervisor as if it was the first operating system on the machine, and then add Windows and Ubuntu as virtual machines under the hypervisor.

Both Xen and Hyper-V will run Ubuntu and Windows as guest operating systems, although it may not be a supported configuration. Xen is more Linuxy and Hyper-V is an MS product, so I'd suggest picking the hypervisor based on whichever OS you're more comfortable with.