USB Windows Installer Winsetupfromusb

http://www.winsetupfromusb.com/
Looks like may be able to have two or more windows install versions on one drive. Worth a try. I have found this is hard to do so will try this if need to reconstruct my flash installers.

To add a windows install to an existing usb without formatting use EasyBCD
http://www.intowindows.com/how-to-make-bootable-windows-usb-without-formatting/
Use the "deploy bcd" button.

EasyBCD can be made portable by installing, copy the directory somewhere, uninstall.