Why is an outward spinning propeller better in theory on the North American F-82 Twin Mustang?
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.