Running multiple Xcode versions
Installing
To use multiple Xcode versions, simply make sure to install from Apple Developer Portal into its own unique folder. I tend to version each folder under Applications as Xcode10.0, Xcode10.1, etc. with each containing the respective Xcode install.
If you previously installed from App Store, make sure you completely uninstall everything and use only manually installed versions from Developer.
Switch command line tools
sudo xcode-select -s /Applications/Xcode10.2/Xcode.app/Running Xcode versions
To easily differentiate between my primary copy of Xcode (usually the latest or one before the latest version) and all others I like to replace the icon of the older Xcode versions to a black and white copy. This let's me easily see which version I am running by glancing at the macOS dock.