There is always some overlap between the two sound categories. Sound Editing is the creation of foley. Sound Mixing is the recording and mixing of the soundtrack. They are two distinct aspects of sound production, but often if a film does one well it does the other similarly well.
That said, a well-mixed film may not require a lot of sound effect production. A musical, perhaps. A film may require a heavy amount of foley, which usually means the mix is harder to master. An action movie, perhaps. A lot goes into the soundtrack, making it hard sometimes to discern whether the Academy will honor films in both categories, one, or none.