Not that I disagree, but if the load is symmetrical, I cant imagine an engine making "the difference" seeing as the engines are not substantially far off from the longitudinal axis. Keep in mind you have an equal and opposite effect from the engine on the other side. I think the the problem is spinning in the direction where the P factor is generated farther away from the longitudinal axis like on a conventional twin. I have spun airplanes with tip tanks, and I have spun that airplane with the tanks empty, and at half, and noticed no difference. BUT I am not going to try it with asymmetrical fuel loads.
We don't know what happened. If he lost an engine, VMC rolled and that turned into a subsequent spin, nothing short of shutting the other engine down, and having a crapton of altitude would help and even that is a long shot. .There have been documented multi-engine airplane spins and recoveries. I am not saying the 421 is one of them, But at 11,000 feet he didn't have much of a buffer to do anything.