Well, I was serious, but the chickies pulled a prank on me!
I retract my post, and humbly admit I've been the naive victim of an early April Fool's joke. One of my chicks seems to have pipped and zipped on the underside of the egg and slipped out while I wasn't in the room, making sure her egg looked perfectly intact from the top view. When I went to clear out the incubator, I discovered my silly mistake. No twins! This makes a lot more sense. I couldn't figure out how two big chicks had come out of that one egg.