Minimum clearance over a combustible surface is 24 inches for a hood and 30 inches for cabinetry. Unless the manufacture of the hood/microwave states otherwise. 18 inches is fine for upper cabinets above plain counter top, but you REALLY need the distance above a range or cooktop to prevent fires.

http://starcraftcustombuilders.com/kitchen.design.rules.htm#rule18_namelink

Honestly, I've had both a built-in microwave (with double ovens below) and separate range... And our current home has an induction stove with an over-the-range convection microwave above it. While I miss my gas range, I think for the space savings (if you're not planning a remodel) you can't go wrong with doing the microwave over the range. If you do have the space, or could raise your cabinet a couple of inches, I will say that our microwave-convection oven combo is the next best thing to double ovens. Oh, and buy an Instantpot if you need double appliance duty with limited space. :-)