Interesting idea. Depending on where the second bomb is dropped, some hawkish members of the Japanese government might try to pass it off as the Americans missing, rather than a demonstration. As to what would happen, it's tough to say. The Emperor's mind was already made up. I honestly think you'll get the same result as OTL. The Soviets are still invading, and Hiroshima's still gone. That's enough to show American resolve in a way that a demonstration strike first wouldn't. The only knock-on effects I can see would be possibly more testing post-war, since you've only got the results of one city blast.