From an anatomical standpoint, I think both arms should retain the rotational functionality that humans have (the whole 2:1 bone structure). As such, this would cause the "at rest" position for the hands to be facing the thigh, not each other. Sure they still CAN face each other, but I think it'd be better if they were allowed multiple positions. I mean, imagine a grey holding the upper hand flat - holding something out - and the lower hand turns flat in the other direction and picks something off the table. Facing each other, countering each other, this would give you a wide range of expressive functions when it comes to orientation of the hands.