The class stories happen concurrently. The exact order of all the class story events isn't completely clear, but it seems to be safe to assume that all the classes finish Chapter 1 before anyone starts Chapter 2 and so on. And really the order of the class stories seems to change on a nearly planet by planet basis. As an example the JK starts Chapter 3 before the SW, but the SW is implied to finish Voss(second planet of Chapter 3) before the JK does.
There really are not enough pieces of data to put together the entire timeline for every planetary event, and honestly it is a rare occurance when you can place two class stories on the same planet chronologically.