Horns were originally manufactured and adjusted to a certain amperage draw for the ******** circuit. The points can be adjusted but a person needs to know what amperage to set it at to insure longevity of the ******** coils and points. Higher voltage will change the current draw. A large resistor can be installed in the circuit to drop the voltage & current back to normal.
Wiring them is series will change the current draw but I don't know if the drop factor will equal what a dropping resistor will do.