Actually, torque can actually assist a turn. The rotary engines on Great War kites were designed to cause the airplanes to turn better in one direction. However, when you throttle back one engine in the P-38, the resulting loss of power cannot make up for the torque gained. Power helps you turn much more than torque does.
I suppose that, above corner turning speed, throttling back one engine might help a bit. But once you're below corner, you need as much power as you can get.