The default SQL Developer 4.0 short cut keys have a few keys that are mapped to multiple operations but the operations are in different categories.
It is not possible to do this yourself via the UI. For example I want to map F9 to both Run Statement (in the Worksheet category) and Compile (in the PLSQL Editor category). Assigning F9 to one removes it from the other and the help text suggests this.
However, I hacked the settings file directly (thus bypassing this logic) and everything works fine. I think it should allow keys to be mapped to different operations as long as they are in different categories.