The singles are a lot like Les Paul's and they have two. The double cuts are harder to put the switch on the upper boubt, I know it can be done, just look at the John Petrucci MusicMan, so it needs to go somewhere else. It can't go on the lower horn as there isn't enough room with out it being in the way so its got to go around the volume controls so you lose a volume and tone to clean things up.
Not all single cuts have 2, some of the older se single cuts have the same lay out as double cuts.