As for Gentoo distributions, for some reasons I need to carry with me different Ubuntu distributions (x86, amd64, live, alternate, etc.) on the same usb disk and to be able to boot and install from them. You (quite) cannot boot directly from the .iso image (with some other distros you can, i.e. quantian). You (quite) cannot put them in a single partition because they all share the same directory structure and jailing each one in a sub-directory did not work. The easiest way is to have each distro in its own partition and a multi-boot scheme for the usb disk.
As stated in the page linked above, it is possible to use a pair of kernel and initrd to load the installer from an .iso image.
If you have several .iso images (related to the same version of the distro) in the same partition, you can choose which one to load from, by renaming the other ones with an extension different from ".iso". The boot process automatically loads and installs from the first image it finds.