I would guess that the ladies are having trouble with reaching the trigger because the grip is too large (Heck, I've got gynecologist's fingers, and I have trouble reaching around a beretta!), and / or they just lack the grip strength period to pull the DA trigger.
Thus, they use both fingers to make it easier.
One of my fellow firearms instructors at work claims that grip strength is THE most important issue with pistol shooting - He will not even bother coaching a weak shooter until they demonstrate that they can pull a DAO trigger 100 times strong hand, and minimum 10 times weak hand. I'm not sure if I lend THAT much weight to strength, BUT... It is awfully tough to maintain a sight picture if the muzzle dips every time you try and squeeze the trigger...
Just my thoughts...
Neal