Thats a very good question. I believe it has to do with capacity. Off the top of my head, Brittania, TWA (The 2 762s with the double-exits actually came from Brittania, originally from Braathens), Balkan (From AF), AF, and Aeromaritime. I believe it's has somethng to do with capacity, or it's just an airlines option.
In order to cram more than the max. allowable passengers into the 762 (by using only the standard four door, two wing exit configuration), airlines were required to install additional exits to comply with the regulatory authorities' requirements.