This is a video demonstration of the bionic Double Hand developed by youbionic. They come in both left and right hand models and you can actually buy one for €1,800 (~$2,120). One hand is controlled by your index and middle fingers, and the other hand by your ring finger and pinky. Based on the video, they don't really seem stronger or more practical than a single human hand, and the functionality seems pretty limited besides being able to buy two and pleasure four robots simultaneously, which, let's not kid ourselves, is exactly what these were invented for.
Keep going for the video.
Thanks to n0nentity, who's holding out for two strap-on bionic arms so he can be more like his hero Goro from Mortal Kombat.