i suppose it depends on the car type and the amount of space available. most coasters that have cars where you can get through it easily just have 1 catwalk.
im guessing it also depends on how many people the train carries. you dont really want a big group of people perched on a narrow staircase 50 foot in the air. it would be best to split them in to two smaller groups for safety.
Sometimes it is simply a matter of efficiency. Lets say you use an elevator to reach the top of the ride, e.g. almost any coaster with a vertical lift. Having an elevator on both sides of the hill seems unnecessary when the job can easily be done with only one. Therefore, you only need catwalks on one side of the hill. This is apparent in most of Intamin's Accelerator Coasters, especially TTD and Kingda Ka.
Don't stall, don't stall, don't stall...
It stalled....
~TFK
