I've been admiring this photography technique for years and only tonight learned the actual name for it. Diptych. It's when two images are placed side by side to show similarities, contrasts, whatever. When it's done with three images it's called a triptych.
How'd I learn it? Apple taught me. No, really. The new iPhone 4 has come out with some kick ass photo apps recently and they just released one called Diptic. Guess what it does? All this time I've been creating mine in Photoshop.