Should one mention multiple, albeit somewhat unrelated research interests?
If you can convey your enthusiasm for the overlap of these two somewhat distinct areas, you will be ahead of the game. A PhD candidate is expected to extend the limit of human knowledge, and one of the better ways to do so is to research within the overlap of two (or more) disciplines. I would tend to see having more than one area of interest as being an advantage rather than a disadvantage. Of course, as your question implies, a multiplicity of interests can backfire by causing a candidate to appear unfocused. Focus on the overlap, and make your application shine!