You don't have to have multiple boot partitions to facilitate the desired functionality.
You can have a choice between multiple bootable images that are on the same USB drive partition.
I've been using
WinSetupFromUSB to create a bootable USB drive with installation images of different windows and linux versions plus utilities like UBCD, various rescue disks and so on, quite handy really. Once it boots it gives you a menu to choose how to proceed.
And you can still use the additional space on the drive for additional storage, just make sure you don't mess with the pre-existing file structure. Put it in a uniquely named folder just to be on the safe side.
There are other utilities that can do the same, but I haven't tested them, as the first one I tried worked out for me.