If you watch pro tennis you'll notice lots of players using a single hand to hit their backhand shots, even if the majority probably use two!
So, we can assume there are pros and cons to both.
But for juniors and beginner players typically coaches recommend using two...and there are specific reasons for that.
Benefits of the Two Handed Backhand
Most new players are taught a double handed backhand because the second hand gives a lot more stability and power to the shot.
When you’re learning tennis, either as a kid or an adult, it’s not easy to have the strength and coordination to do this effectively, and this is where having the second hand on the racket makes a huge difference.
Another significant advantage of the double-handed backhand is hitting balls that are above the waist, which are much easier for a double hander as single handers tend to struggle with balls above the waist due to the lack of control.
A good player will take advantage of this weakness and tend to hit loopy balls to a single hander, but this tactic will not be useful against a player with a good double handed backhand.
With a two-handed backhand, a player tends to get away with a poorly timed shot and recover more easily than a single hander would which is another huge bonus.