Interesting observation. I have only seen it on youtube videos. Looked a little goofy and lobsterish to me.
I think additional benefits would include:
Developing ambidextrousness (is that a word?);
Finding a way to make it difficult to get balls out of your pocket or even feed a ball;
Never improving your backhand;
Looking a little "out there" on a tennis court;
I bet Marion Bartolli might be into something like this. She seems to shun convention, and embrace the unorthodox.
Not trying to criticize. Anyone who wants to use two racquets should do so. Maybe the racquet manufacturers should get on board with this. They could double their sales!