Many DC systems were 3 wire "Edison" systems. This required two 110V DC generators connected in series. The result was very much like our standard 120/240 3 wire service of today. The 2 generators in series would create 220v on + and - outside lines with the center line that was tapped to the series +- connection between the two generators having 110v to either line wire. This allowed larger DC motors to be run off of 220v such as elevators and still have 110v for lighting. I believe additional pairs of series connected DC generators could be connected in parrallel, with little difficulty. With AC this becomes slightly complex as each additional generator that is connected to the grid has to be syncronized so that the wave form matches. This was done manually in older AC generation with syncronizing lights and or syncroscopes which aided the operator to close the circuit breaker at precisely the right moment. Modern electornic equipment does this without chance of human error. Years ago I was allowed to observe at a small 1.35MW hydroplant that still had to be sycronized manually. The operator who was about to close the breaker was doing so under the eyes of the head of that utility companies hydropower division. He missed the moment and the high inrush current due to the generator being out of phase caused the breaker to trip with a rather loud bang that got everybodies attention. I imagine folks living near by had a pretty good blink in their lights. The second time around the operator got it right.