Never knew that exists. I doubt it improves braking because your finger force will be split in half for each brake. I guess diverting braking force over 2 brakes helps cooling each brake, though. So the above tandem comment makes sense.
I first though it was for a person with just one hand to control front and rear. but on a bike, you want separate control over rear and front brake (unlike a car, where the driver only has 1 brake, but the computer uses electronic brake force distribution, ABS and VSC to control each of the 4 wheels individually)