It is possible to install multiple versions, and it is also possible to install over a previous install without uninstalling first. However, in cases where we're troubleshooting we like to recommend only having one version installed and to uninstall first, just to rule out any installation weirdness.
If you want to install multiple versions to multiple directories it's probably best to manually copy the program folder for your older versions, just in case the installer automatically removes the previous installation before installing its own version. Of course you are then responsible for that copy of the program folder, as when you come to uninstall it would only remove the most recently installed folder.
Matt