Quote -  try this on for size.

put element "A" on a layer    
put element "A+B" on another

change the blending mode of one of them to "difference"

flatten, invert the colours.  

Thanks. That's the kind of answer I was looking foi. A bit incomplete, but I worked out the rest from there. Doing this turned the apple a solid color, which made it easy to crete a layer mask showing only the banana

Quote - I
Can you post a pic of the actual image you're working on?

As mentioned in the OP, I don't HAVE a specific picture I'm working on, yet. This is more something that would be good to knoiw for future. Difference blending seems to do the job well.