We have the Monogram 120 Advantium with the Trivection and Warming Drawer below. Haven't used yet as remodel is not yet complete.

My daughter did what you are planning on doing. I’m not sure her gap was exactly the same size and her ovens were only 24” wide. Since she did not want to remodel her whole kitchen, she stuck with 24” wide replacements. She put the oven on the bottom and used a speed oven/microwave on top. Like hers your microwave will need a shelf to support it. She had a carpenter put in a shelf and a filler piece between the lower oven and the upper. She also needed an electrician to put in an outlet for the microwave/speed oven. Her over the cooktop microwave was replaced with a vent hood Good luck with your project.

I have a microwave on a counter-height deep shelf in my pantry to hide it. We even have an ice maker and wine fridge below the counter. this setup works just fine. We also use our microwave several times a day. Our pantry doors are open a lot too. i have a Breville Smart oven that sits on the counter. It acts as my toaster and second oven for casseroles, etc. the oven is on the larger side, but I put it in a corner which makes it better. i agree with not putting a toaster or even a toaster oven in the pantry because of the cleaning issues.

In my old oven/mw combo unit, the mw died after two years, but the oven kept going for another 15 years. I didn't want to experience that again, so when the oven finally began to fail, I replaced them with separate units. It was easy to build a shelf to make a mw cubby above the new oven. The expense was in creating an electrical outlet & redoing the wiring. IIRC, it was about $2K to run wiring. We also created a new breaker box nearby for the oven & mw, so that added to the costs. Better infrastructure is always worth the $, imo. My new oven is a GE Profile Twin Flex: two small ovens in the space of a single. So nice to have a double oven now after only having one previously!