If I have 2 copies of "The Defense of Yavin 4" in play. And want to deploy a vehicle, for each card I discard from my hand does each card then reduce the cost by 2, since I have 2 copies of The Defense of Yavin 4 in play? Or can't I combine these and therfore each discarded card from my hand only reduce the cost by 1?

No.

Each card you discard corresponds only to one activated ability.

You can't combine them because the discard you're doing is playing a cost. It would be like focusing 1 resource and playing 2 1-cost units with it.