The way iSplit Photo works is that you take of photo of one friend's face, then you lock half of it (the top half for example). The app then lets you take a photo of another friend's face and line that up in the other half of the photo.
The app then stitches the two photos together and spits out a photo of your two friends' faces combined.
It's really easy to use and the results are both remarkably good and hilarious.