I could use the keyboard and the mouse while scratching my head all at once.

Get half as terrible at juggling.

Well I would only assume that I would be a much better rock climber.

Solder without alligator clips

Use the second set of arms to restrain the first set from sexually harassing me

Now I truly never need to make a second trip back to the car for groceries again.

I can finally constantly hold an umbrella

Will the first thing would be to Google how to date 2 women at once.

oh man I would have to start practicing sleight of hand again I'd be the coolest magician

I can quit my job and become a mega cashier at a grocery store. I'd be unstoppable.

Learn to play the Antagonistic Undecagonstring from The Hydrogen Sonata by Iain M Banks.