As we know that, for zero order reaction the rate of reaction is constant and does not change with the concentration of reactant. So, doubling concentration of reactant a will have no affect on the rate of reaction. However, for second order the reaction rate is proportional to the square of concentration of reactant. Hence, if concentration of b is doubled, ration rate will be quadrupled. So, overall reaction rate will get four times its initial rate, if concentrations of both reactants are doubled.