I too find that practicing a two-handed stroke improves the one-hander. I always had a one-handed backhand. It used to be my better shot. But when I developed a big forehand, it atrophied because I began running around a lot of backhands to unload my big shot.
One day, just for the heck of it, I decided to see what a two-handed backhand feels like, so I started hitting some against a wall. I found that when I then let go, my one-handed shot was better. My preparation and footwork were better, my swing was straighter -- everything was better! To this day, when I start missing backhands, I hit two-handers for awhile to straighten my backhand out.
As for a two-handed forehand, I think it helps to keep in mind that you drive the
racket through with the hand behind. For a right-hander, that would be your
right hand on the forehand side and your
left hand on the backhand side. So, on a two-handed forehand, you're still
driving the
racket with your
right hand, not pulling it with your left.