On the one hand, this is relatively easy. If you have a PC that boots from USB, you just have to set up the USB drive to multi-boot the way that you'd set up a HDD to multi-boot.
On the other hand, I can't think of a single good reason to multi-boot different versions of the same OS. What is the advantage to have Ubuntu, Fedora, etc., ... why not have a single distro that boots different configs and kernels (if necessary)?
I did not know that was possible.
In certain situations I need certain OSes, which is why I wanted this setup.