I don't think it's possible, because on page 39 it explciitly says so. However, my friend argues that you can. His argument is this: the Monk's unarmed attack discription says you cannot make any off-hand attacks with fists. He says the ambidexterity feat gets rid of your off-hand, because you are good with both of your hands, and that, therefore, you may use twf with fists. We do play 3.5e, where ambidex doesn't exist, but he says the underlying principal is the same.