Combo units are troublesome. The MW will probably die long before the oven. Then you have a very expensive repair (if even possible) or replacement. Better to do either double ovens or separate oven and microwave units.

Combo units are expensive, and expensive to replace. Microwaves will eventually die. Replacing just a microwave is easy, and inexpensive. Better off getting a double oven, and separate microwave. You can put the microwave on a shelf, and get a kit to make it look built in, or simply install it on a shelf like this: