If Mihawk lost his arm freshly, two arms Mihawk would beat one arm Mihawk high diff. It's not like one arm Mihawk's power level drops by 50%, since his other stats, especially Haki remain the same, just as his speed, reaction, hawk vision, etc. You also have to consider that with years passing, Mihawk would greatly adjust to his handicap and honing his skill with one arm.
And as it was pointed out, Shanks' sword is better suited for one arm usage than Mihawk's humongous Kriegsmesser.