Assuming you could succeed at get both hands to hit exactly over his ears in the right fashion, the most you could to achieve is making his ears pop painfully. However, even succeeding at such an attack would be really difficult unless he was perfectly still and unaware of you.

Try a swift jab to the solar plexus, a kick to the groin, headbutting his nose; or better than any of these, try running away. If you seriously don't know a lot about fighting, running away is by far the most intelligent thing you can do, and it's great exercise to boot.