At some point you have to rely on your talent. A skilled OF uses one hand. By raising the other arm you cut down on mobility when tracking the ball. Yes even the most skilled players drop balls.
Fielding a ground ball is quite different than catching a fly ball. Ground balls you set up glove side foot is slightly ahead of bare hand foot. You bare hand is down ready to trap the ball and or knock it down if it moves away from the glove. Fly ball your glove side is more forward allowing you to site better and catch the ball over your throwing side. Your bare hand comes up to transfer the ball and your body is positioned to shorten the cocking of the ball for the throw. Your feet have to be in a position ready for the long throw.