One of those two wires will be a switched hot 12 volt source (such as from the ignition switch).
The second wire will be the tachometer pulse signal. This wire comes from the ignition coil and sends "square" wave pulses which the tachometer translates into RPMs. Some ignition coils use a "current" pulse, meaning it is a hot output to the Tach., others use a "voltage" pulse, meaning it is a ground output to the Tach. But, the replacement should be compatible with your ignition coil :mrgreen:
-Nick :!: