My advice would be not to get hung up on this idea, but rather focus carefully on each deal, and don't lump them together. You might, before or after the negotiations are done and on paper, ask for maybe some free floor mats or something.
So in other words, hammer out your best deals. I doubt the dealer really cares if you buy one car or two. To him, each sale is its own thing. Any little perk will probably come from the salesman's pocket, not the house's.
Of course, you can ASK for a multi-car discount, but after all the smoke and mirrors have been brought out to impress you, I doubt you'd end up with any more or less than if you had bought one car.
So stay focused on each car's deal.
You can use Edmunds TRUE MARKET VALUE
pricing guide to help you set yourself a "target price" for each car before you go in.