Two-racket tennis as a training method can (and should I believe) be used by all types of tennis players (recreational, amateur and professional) hoping to improve motor skills and muscle power for both sides of the body.
Let's take the garden-variety pro player as an example: For the player who uses the two-hand backhand, which can be thought of as a choked up forehand, training the opposite forehand will bolster the two-hand backhand both in power and control. It certainly hasn't hurt Nadal; a natural righty who was persuaded to play lefty.
What about the player with the one-hand backhand? That player would also benefit from opposite forehand training. As one learns how to play an opposite forehand not only is a skill being created in controlling the swing of the racket, but also in the control of the body parts, which go into the completion of the swing. For you one-hand backhanders out there, I invite you to try playing an opposite forehand for a few weeks. Then go back full force to your one-hand backhand. I bet that you will see improvements in both power and control of the backhand from having developed this new skill. You will also find that you have increased stamina and perhaps even a flavor for creating shots that you normally would have been afraid to try. Try the two-racket test. I think your game will profit from having done so. Enjoy…
Professor Tennis - The Physics of Tennis