Playing with two rackets trumps the Two-hand backhand.
As a physics professor (aka Professor Tennis) I see three inherent flaws to hitting with two hands on the racket:
(1) Placing a second hand on the racket further reduces the player's range of motion. Simple geometry here: Why would a player stretch for a ball with two hands on the racket when stretching with one hand allows for a greater stretch? Perhaps the weakest shot in tennis is that of the player who tries to return a ball, which is at the limit of his/her ability to reach using a two-hand backhand. In this instance, the player simply gets little on the ball. The single-hand return gets more power on the ball and affords for enhanced slice on the ball.
(2) Having two hands on the racket for both forehand and backhand returns suggests that the player needs to switch the placement of the hands on the racket handle when going from the forehand to the backhand shot. A perfect example of this is in the hitting of a baseball: The right-handed batter places his right hand above his left hand on the bat handle, whereas the left-handed batter employs his hands in the reverse order (i.e., the left hand above the right hand). This is the proper way to hit a baseball. This placement of the hands on the racket handle would also be the proper way to hit a tennis ball. Consequently, there is a switching of the hands when the correct hitting method is applied. (Of course, tennis players are known to adopt foolish techniques in simply not knowing any better.)
(3) Whenever a player uses two hands on the racket to return a shot, the player must "cross over" his/her body. The cross over approach to hitting a ball, whether it be in baseball or tennis adds increased strain to the lower back as the player turns to initiate the swing. (A price paid by former Yankee great Don Mattingly and his bad back; keeping him out of the Baseball Hall of Fame.) The older we get the greater the risk of straining our lower back when we do the cross over swing.
Some "Physics of Tennis for Thought." Enjoy.
Professor Tennis - The Physics of Tennis