I would start by connecting them together and using both. On most circuits, you get 12 volts on the low-speed wire first, then, when the high-speed is needed, that wire gets 12 volts and the low-speed wire drops to 0 volts. With your original two-speed motor, you don't want 12 volts on both wires at the same time as that would, in effect, short out part of the motor. If that is how your car works, and you only use the low-speed wire, your new fan will run at full speed at first when the fan is first needed, but that circuit will turn off when the high speed is needed. That would result in overheating at low car speeds. If you only used the high-speed circuit, the engine would have to become undesirably hot before the fan turned on. Eventually you could entice a warped cylinder head and leaking head gasket.
If tying the two wires together causes a problem, I can draw up a circuit using two relays to run the fan motor. You will not damage anything, but the Engine Computer on your car might monitor these lines for proper operation, and it might detect voltage on one wire when it should not be there. I do not know that for a fact, but if it were to occur, I will have a solution.
Wednesday, March 22nd, 2017 AT 7:44 PM