In tennis, rule #24, which covers all the scenarios in which a player loses a point, does not forbid a player from switching the hand that holds the racket.

How hard is it to switch hands in tennis?

To learn to switch hands, it takes years off your game with your good hand, and most ambi hitters never ever learn to play a serious match using both hands. If you're prone to tennis elbow with one hand, you WILL get it using the other hand, just need equal time hitting the ball equally hard.

Are you allowed to use two hands in tennis?

Yes, you absolutely can hit a forehand with two hands. Monica Seles did it for years and was tremendously successful. Hitting a forehand with two hands isn't nearly as prevalent as hitting a backhand with two hands, but it does occur.