You can install more than one version of Unity on your machine as long as you follow the correct naming conventions for your folders. You need to rename each of the Unity folders themselves, so that the hierarchy looks like:
Unity_3.4.0 ---Editor ---MonoDevelop Unity_4.0b7 ---Editor ---MonoDevelop
If you rename each Editor folder inside a single Unity folder you will overwrite the MonoDevelop folder and this will cause serious stability problems and unexpected crashes.
Page last updated: 2012-09-20