I have Xcode 6.4 and want to update to Xcode 7. But I want to keep the 6.4 version working and have both as well. Does this work? And if so, how? I am running OS X 10.10.5 if this matters.
You should be able to do such a thing by renaming Xcode.app in Xcode6.app from the /Application directory then download Xcode 7 from the App Store.
Edit: Thanks to the feedback from HannaB, we need to copy the Xcode6.app somewhere else before downloading the new version otherwise it will overwrite the current version. After the installation of the new version, you can safely move Xcode6.app back to the /Application folder