It's possible to do this through grub2 - either booting into different partitions or simply booting the ISO file directly. It's also possible if you have a large enough USB drive to partition it and do a full install of both operating systems on it. If you do a search you'll find a lot of information about these things.
Unless you want to do these as a learning exercise however I'd suggest the easiest thing to do would be to use MultiSystem to configure your USB drive with as many operating systems (and types) as you wish. Go here:
and download the installer file install-depot-multisystem.sh.tar.bz2
You will then need to unpack and install the program, then run it to build your USB drive. You don't need to download the ISO files again - multisystem just defaults to that mode of operation. Pay attention to creating a persistence file since it boots from a virtual disk. It's not difficult as everything is automated for you.
MultiSystem also works for the Gparted LiveUSB and other recovery tools so it's great to build a complete recovery USB drive with, especially those that aren't designed to boot straight from the ISO (like the Gparted CD).
You do however lose the first test and repair menu of the LiveUSB's, since multisystem causes it boot straight into the operating system. You can certainly add these tools back into the grub2 menu that multisystem creates however.