Those are your top gears and both should be held onto the top shaft with a roll pin from the factory. If either of them spins freely, you broke the roll pin. The answer to this is either a drill bill shaft or a allen wrench shaft cut to length and put in where the roll pin went. You can search here a bit and find lots of info on how to do this.
Edit: Somehow I didn't scroll to the bottom of the thread and see that Finnster had already answered this. Oh well...