more valves than stations
Whatever method you use a new controller will be required. The splitter doesn't have a brain of it's own.
If each valve has wire returning to the controller it may be best and easiest to install a new controller.
Check to see if performance is significantly better when only one valve runs before doing any work. Flow should generally not exceed 10 GPM with the size pipe and valves your working with.
If the wires are joined in the field and not at the controller a splitter may have to be added. Splitters I've used simply had a electro magnetic switch inside. Each time power came on and off it would automatically flip to the opposite wire. You still must have a controller station for each valve but do not need a wire for each valve. That will still necessitate a new controller.
If valves are located close and convenient to the controller, replace the controller and add the required wire to the valves.