1st - You can'r really RUN both simultaneously on the same machine - they can be installed - but running them at the same time requires a very advanced knowledge and a lot of workarounds - fixes. Short answer - don't do it.
To easily work with the thumb drive - You need to make a shoertcut for the XAMPP Control Panel on the thumbdrive on your desktop.
They will BOTH have the same address -- http://localhost
--(as they are a webserver running on the loopback address of whatever machine they are on)