Unfortunately, it is currently not possible (at least to best of my understanding). When your computer starts, it runs your boot loader. There are many boot loaders that support loading multiple operating systems, but unfortunately none of them can load more than one a time. I don't even think it would be possible for the boot loader to load more than one at a time. How would they manage resources (RAM, Display, Hard Drives, etc.)?
However, I do have an idea for how your question could be achieved. Who knows, maybe I'm just not aware that someone has done this yet? But any ways... What could be done is a to develop a single lightweight operating system that is nothing more than a virtual machine manager. Then you could install your different OSes, and switch between them rapidly.
A quick and dirty solution to this might be to install a very slim Linux distro (Gentoo for example) and install Virtual Box, create some virtual machines and install your OSes, and have the Linux distro load the virtual machines in full screen mode at boot. Just keep in mind that 3D support for Windows in the virtual machine will be extremely difficult to achieve.
Another option might be two PCs with a KVM switch?