The reason we apply two coats is usually because the first coat soaks into the surfaces unevenly. The first coat fills in the low areas so that coat #2 looks more even with an even sheen. It also provides greater durability. I'm not a paint chemist by any stretch, but in my view film build is always better achieved by applying multiple coats with drying in between than applying one heavy coat.

Would that be acceptable to you? As long as the mil thickness exceeds standards, does it matter if one or two coats are applied?

No.

I don't know what paint he's using or what kind of magical sprayer he has but the first thing I'd be worried about is sagging.

The dry mil thickness gauges that I have read about only work on steel or metal surface. How is a homeowner supposed to determine the thickness of the paint that was applied to the exterior of a house? Whether it's one thick coat or two thin coats, how is a homeowner supposed to know how much paint was applied to the exterior of a house? I will be going from a darker color to a lighter color, so I want to make sure enough paint gets applied to cover up the darker paint to where it won't fade or bleed through in a few years. How can I as a consumer ensure this when hiring a painter?

Each exterior painting project is unique. For example, if you are doing a "maintenance coat" of the same color, or painting a color a few shades lighter or darker on pre-coated surfaces with solid integrity, one coat sprayed or rolled may suffice. When changing from light to dark or vice versa, changing sheen, restoring a surface, or coating a new surface, applying multiple coats is important to create surface resilience and "build" or mil thickness. Purchasing quality paint or stain will help give longevity to fading, but all paint fades over time. As far as tannins or knots bleeding through on wood, it's important to use proper shellac or oil primer on bare wood to help prevent this from happening.