I have them installed side by side right now and I have not encountered any problems with this arrangement. While I obviously haven’t had Xcode 6 for very long, I have done the same thing with several previous Xcode beta releases as well and I have no reason to believe this release would be different.
EDIT: You can use xcode-select
to switch between the different versions of the command line tools. From the man page:
When multiple Xcode applications are installed on a system (e.g. /Applications/Xcode.app, containing
the latest Xcode, and /Applications/Xcode-DP.app containing a developer preview) use xcode-select
–switch path/to/Xcode.app to specify the Xcode that you wish to use for command line developer
tools.