Hello Scientist;
As far as I know, LAX has never had two operating control towers. They have a new one and an old one <G>.
DFW has three towers but use only two of them at a time. The center tower was used for midnight operations but I believe that they staff the two outboard towers now for the midnight shifts... I am sure that you can find someone at DFW who could answer your questions... The main reason that they built two towers though is the size of the airport and being able to see it all.
As for OAK. One tower is for the comercial ops and the other one is for the General Aviation side. They both use different runways.
regards