I've never needed to know, in more than ten years of answering official GMAT Quant questions, that when a rectangle's diagonals are perpendicular, that rectangle is a square. But that's what this question is testing. If you know that, Statement 2 is clearly sufficient alone, and Statement 1 is not (the diagonals of any rectangle are equal in length by Pythagoras, so Statement 1 is always true and doesn't give any new information).