Nothing will happen. It's a myth. The myth of the one-page letter is so strong that I have heard people ask this question over and over again. Yes, it's true, a one page cover letter is what is considered to be a norm here, but if you have a good reason for exceeding it, I assume nothing will happen. Of course, it differs from place to place. Someone might just disregard it, looking at you as unprofessional because you exceeded the limit that they gave you, which was a test in itself. So it really depends more on the place. Hope this helps
