China may double the value of the dollar to its makers of export items like watches, sewing machines, and radios. Peking needs overseas trade to cover its growing needs for foreign technology and machines. But with the Chinese "renminbi" officially valued at around 1.46 to the dollar, Chinese industry can sell its products more profitably at home. So as an inducement to find markets abroad, manufacturers have been told privately that after Jan. 1, 1981, every dollar they earn will be counted as 2. 80 renminbi -- nearly double the official rate.