How to create a USB installer for multiple OS on an OS X system?
The way to do this is to create multiple partitions on the USB drive.
There is a Tech Republic article that gives the full details on how to do this:

http://www.techrepublic.com/article/pro-tip-create-an-os-x-multiboot-usb-install-drive/

In summary, TechRepublic gives the steps:


Using Disk Utility, create separate partitions for each version of MacOS that will be booted. (Mac OS Extended (Journaled) format; In "Options" choose "GUID Partition Table.")  It's a good idea to name the partitions for the Mac OS version you'll be booting.
Copy the relevant contents to the particular partitions.  This is different for different versions of MacOS:


Apple OS X 10.7 (Lion), 10.8 (Mountain Lion):
a. Right-click the installer and select Show Package Contents from the context menu;
b. Drill-down through the Contents | Shared Support directories and drag-drop the InstallESD.dmg to the desktop;
c. Using Disk Utility's "Restore" tab, Perform a Restore into the appropriate partition, using InstallESD.dmg as the source.  This takes about 30 minutes.
Apple OS X 10.4 (Tiger), 10.5 (Leopard), 10.6 (Snow Leopard):
Since these were distributed as DVDs, you restore from the DVD or ISO image. (DVD restore will be much slower than the file restore method.) Use Disk Utility's "Restore" tab with the DVD or ISO image as the source.
Apple OS X 10.9 (Mavericks) and 10.10 (Yosemite):
Ise the new "createinstallmedia" command. If your installer.app is not in /Applications, modify the "-applicationpath" argument appropriately.)
For example:  


sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia —volume /Volumes/DRIVE_LABEL —applicationpath /Applications/Install\ OS\ X\ Yosemite.app —nointeraction





I expect that the method for 10.7 and 10.8 will work for later versions of MacOS as well.