Actually I just now tried renaming the superfluous copy of EditorGUIUtility, and I got the same error with InvDatabaseInspector.
Starting a new scene doesn't seem to help. Is there a particular directory in the project these files need to reside in, or something?
The other Asset Store content I'm using is Ultimate FPS Camera by VisionPunk. I don't know where that developer may have left remnants of his copy of NGUI, so I don't know how to uninstall it.
Okay, I just created a new project and I still got those errors. Deleting one makes the other work. Unfortunately, I need both.
Is the problem caused by the fact that there's more than one folder named Editor? If I move all the scripts into the same Editor folder, and delete the extra one, will it start working? This page seems to suggest that that's not how it works. But I can't think of any other reason why the scripts would be preventing each other from compiling.
Any thoughts? NGUI and Ultimate FPS Camera are both pretty popular. I'd be amazed if I were the first person to try using them both in the same project!