Why can't they both lose?

Harry Truman on Hitler and Stalin:

• If we see that Germany is winning we ought to help Russia and if Russia is winning we ought to help Germany, and that way let them kill as many as possible, although I don't want to see Hitler victorious under any circumstances. Neither of them thinks anything of their pledged word.
  • As quoted in The New York Times (24 June 1941); also in TIME magazine (2 July 1951))
   

Whenever two people I hate/don't like/can't bring myself to say anything positive about start fighting each other, I have a similar thought: not that I want to kill as many of them as possible, or even want them dead, but that I'd like to see them both "lose."

Case in point.