Does a sprinter use both legs out of the starter blocks? Of course they do. The push off should be with both legs, flexed, to get the maximum drive of the stride leg, up and out, towards the catcher. The bigger stride will generate additional speed. Another advantage is the ball is released a little bit closer to the batter making the batter have to react a fraction of a second faster to make contact with the ball.
With that said, I would focus more on a) big, fast vertical arm circle and b) internal rotation arm whip to generate the most speed of the pitch. IMO, those two areas generate 80% or more of the speed of the pitch.
