I would like to try out different Linux distros on the go using a USB flash drive.
Is it possible to install multiple Linux OSes on a single USB drive?
There are several options available
You could try MultiBootUSB downloadable here
MultiBootUSB is available on SourceForge and appears to be a pretty popular program for just this kind of task. The MultiBootUSB software is also a portable application, meaning you simply put the MultiBootUSB-6.4.1.exe file onto your USB drive and run it all from the USB drive.
You could try XBoot, it has considerably more documentation that MultiUSBBoot, although not really needed it's nice to know it's there, it's downloadable Here
The speed with which XBoot added the ISO files was also blindingly fast – mere seconds per ISO – versus the hour it took me to load the same ISOs into MultiBootUSB.
Downloadable here - I don't have much experience with this tool, so I'm not going to say anything about it.
For your consideration, there is a great MakeUseOf article on this topic, located here
There's a tool that can do just that: http://www.pendrivelinux.com/