Oil Sending Unit Wiring

I'm assuming that you're working with a '66 Barracuda instrument cluster, and you're just asking about the oil pressure sender? If so, it would probably depend on the specs of the stock sender vs. the aftermarket one. The sender is really just a pressure variable resistor to ground. I don't think you'd be able to run both gauges off the same sender however, unless you could figure out how to isolate them from each other so they don't allow feedback that would act as a ground and give you an incorrect reading. I think I'd just add a "T" and the extra correct sender and wiring for the aftermarket gauges. Same goes with the temp sender. Might be a plumbing job getting everything to fit though. If you're working with a car with an idiot light, you'll definitely need the correct sender and wiring for the aftermarket gauge(s).