Lord of the flies: The twins sam and eric?

Though i've read the book, i don't really have a consistent knowledge of the characters Sam and Eric. It's obvious that they're twins and are often refered to as one person, however i'm finding it difficult to put them both in specific themes? (not individually) For example one may be that they are both inseparable. Yet apart from that i don't know the characters well enough to place into certain themes! Can anyone help?