So this came up as a question earlier this week after a game. There is nothing prohibiting two Orbital Resupply Stations from being out in play. But if you manage to get them out, does it then reduce the cost for each Imperial Navy vehicle with a cost of four or more by two?

I would say yes

That is correct. There is no limit or other thing preventing that from working.

It is perfectly legal and has the funny side-effect of making an AT-AT cost 2, while the AT-ST's from Endor Gambit still cost 3. I've had it happen.