Generally speaking....some of it has to do with staffing, and some of it has to do with the airport layout. If 2 ground controllers are going to constantly be in eachothers' way, they may have just decided to run the operation with one controller...that's the reason why the local controller may 'own' some taxiways; adding an additional cook in that part of the kitchen may help with frequency congestion, but they will be making the operation more complicated. Any time 2 controllers are sharing pavement, the opportunity for error increases.