They are the two half of a whole. They were together even before they were born. Rare are those who have seen one without the other. The little they did experience alone, they told everything about to the other. Even their memories were alike.
They eat the same things, do the same things, and by the way they always finish each other sentences, I'm pretty sure they think alike too. Can they really be considered two different persons? Sure they have some subtle differences in preference, habits and even some physical ones, but so minim that, unless you know them ... thoroughly, they are indiscernible.
They are so alike, is it really so unexpected that they would fall for the same girl? They shared so much, why would it surprise me that they would be willing to share her? Is it really extraordinary that the girl would love them both? They are so identical that it would seem utterly unfair to choose one over the other...