I'll have to do some research myself, but we need Amy to clarify about the "two" eggs versus the "one" that splits.
I've always heard that fraternal twins are two eggs that have been fertilized and each have their own sacs; and, identical twins come from one egg that split and share the same placenta. But, Amy said identical twins can have their own sacs, so...
Added: Sorry, Amy, I was writing this and posted after you did yours. Thanks for the info. I figured if my sister and I ever wanted to know for sure, we'd have to do DNA testing.