Why is an outward spinning propeller better in theory on the North American F-82 Twin Mustang? In this question the Wikipedia article of the F-82 Twin Mustang is quoted. In the third paragraph of the article it says:


  In this arrangement both propellers would turn upward as they approached the center wing, which in theory would have allowed better single-engine control.


There was a comment in the other question that questioned this statement:


  according to that article, an outward-spinning engine configuration is better in theory. i don't see how this would be. in a single-engine
  scenario, wouldn't the working engine torque push the non-working
  engine side down?


A spinning propeller should also produce more lift on the wing section behind it as it is increasing the airflow over that section. Forgetting about the torque for a second, this would already push the other wing (with the failed engine) down.
[EDIT: As answered in the comments, the lift generally has way more effect then the torque.]

So what exactly makes it more favorable to have a outward-spinning configuration in theory?

Also: Does this only apply to this aircraft, or is it valid in general?

Why does it say that the outward-spinning engine configuration is better in theory? What am I missing?
The theory is actually quite simple and straightforward. It should go like this:

In one-engine flight, the asymmetric prop position will pull the aircraft into a sideslip. The sideslip-induced rolling moment will then proceed to roll the aircraft such that the airplane rolls over the dead engine - something every multi-engine pilot learns to avoid early, because it will be hard to get the airplane back to level flight in this configuration.

If the single operating propeller will give the center wing an increased angle of attack due to the propeller swirl, and the outer wing a lower angle of attack, the prop produces a rolling moment which works against the sideslip-induced rolling moment, and should make it easier to avoid rolling over the dead engine. Engine torque will work against this moment, but the induced rolling moment of the wing is stronger, leaving a net moment which rolls the aircraft over the operating engine.

This should apply to all multi-engined aircraft. My reasoning why it didn't work for the F-82 can be read here.