The simple two-finger roll was often used by Earl Scruggs to play melodies on the bottom (4th) string. While you play the melody on the 4th string with the thumb you play the open 1st string with the middle finger in between every note. This technique is very useful for playing quicker sections of melody, and is usually blended in with Square Roll and the thumb/pinch combination. Note that this is the most basic possible 2-finger roll and it is possible to adapt and enhance this technique to produce a complete playing style called 2-finger picking.
Links to examples of songs