Nitrox, doubling is caused when the shooter allows his finger to hit the 2nd trigger inadvertently. However, that does not need to happen when the front trigger is pulled as long as the shooter maintains a firm hold on the pistol grip and control of his fingers. I did not mean to imply that the trigger finger is immoble. Just that it does not pull the trigger until the shooter is ready.

Pulling the rear trigger second is faster than pulling the front trigger second because pulling the rear trigger after recoil is a natural action. But pulling the front trigger after recoil is a movement against the recoil, which is a non-natural reaction and thus slower.