Typically you actually use more than two sensors -- six or eight are good, for line following. The further out to the side you detect the line, the stronger you make the turn signal.
Basically, when you "detect" that the sensors on the right hand side to "dark," you have a crossing to the right. If you want to turn right, start turning until at least one or two sensors on the right are "light" again, while moving forward. Keep doing that, and the 'bot will turn 90 degrees as it moves forward.