If you need to be running Xcode 7.3.1 and 8 together (as I am!); you can not install them just like that. You need to fiddle a bit with the apps to have them work together.
- Download the Xcode 7.3.1.dmg file from apple developer portal.
- Download the Xcode 8.xip file from apple developer portal.
Once you’ve done that, you can install version 7.3.1 into “applications”:
- Double click the .dmg file to open the “drive” it represents.
- Drag the Xcode icon into “application"
- Rename the resulting xcode.app in “applications" to xcode_7.app
Now - the turn comes to Xcode 8. You unzip the .xip file and drag into Applications the resulting xcode.app file (it’s version 8).
You now have 2 versions of Xcode on your Mac and start them independently.
The reason I’m not renaming the XC8 version is due to future updates coming from Apple. I’m not sure how they search for apps locally during an update process...