I have published several papers on what are called discovery machines or creativity machines. See, for example, Trans. Kansas Acad. Sci. vol. 102, pg 32, 1999 and my book Twelve Papers (www.robert-w-jones.com under "book") In the last few years my artificial intelligence A.s.a. H. suggested that a hand with 2 thumbs might be better than a hand with 1 thumb. Often times one can not tell how a creativity machine comes to its conclusions. The same is frequently true of human creative thought. In this case, however, A.s.a. was using its extrapolation algorithm. It had been told that a hand with a thumb is better than a hand with no thumb. (One might challenge this, of course, even though many people have stressed the importance of the opposable thumb.) Extrapolation evidently led A.s.a. to suspect that two thumbs MIGHT be better still. (A.s.a. knows to treat extrapolations as uncertain postulates.)
We can, of course, build robot hands having two thumbs. In fact there is a continuous spectrum of possible geometries. (You can google search "robot claw hand" and find quite a few.) A simple pincer made of two opposed "fingers" might be thought of as one using just their thumb and forefinger. Going beyond a hand with two thumbs might get one to the usual "claw" geometry, i.e. 2 pincers oriented at 90 degrees to one another. The number of fingers can be varied in each geometry. So the question is, which geometry is best? I suspect there is not a simple answer to this. Probably the question is, best for some particular task and environment. Perhaps we can get an engineering student to work on this.