Creating a dual boot USB drive with multiple Linux distributions
Step 1 - Choose your USB Drive
Step 2 - Choose your first operating system
Step 3 - Browse to the ISO
Step 4 - Click Create
Step 5 - Wait for the files to be installed
In theory you can install as many operating systems as you desire but it depends on the amount of disk space of the USB drive.
The end result
If you choose the USB drive you then get to choose the second menu which lists all the distros that you have installed.