If you don't need the speed oven to be a speed oven, and you have room for both an oven and a microwave, the separate units, especially if the MW is a counter model on a shelf, will probably cost less. If you use the MW function on the speed oven a lot, it'll degrade as all microwaves do, and you'll have an expensive replacement in the not so far future. And, you, yourself, said that when you use two ovens at a time you'll want a microwave.

I do use my speed oven as a spare oven and microwave more than as a speed oven, but when I chose it, I was intending to use the speed features more. Most importantly, when I'm using it for entertaining as a third oven is the last time I would want a microwave and I've never had any conflict that way. That's me. You would. Good reason not to do it.

I put in a Wolf oven on the bottom and a Advantium speed oven above - I am pleased with both so far (and keeping my fingers crossed that I don't get the dreaded Wolf porcelain chipping)

I use the Advantium as a MW, speed oven, warming oven and occasionally as a convection oven. The only trick is timing if you want to MW after speeding as the sensor won't work with the oven too hot (you can still MW but not use the sensor function)

Last night - I forgot and started to use the keep warm function and then remembered I needed to defrost and possible MW some frozen mac and cheese (that I made in my Vitamix - well the sauce)

I just discontinued the keep warm, swapped out the metal tray for glass tray and then used the defrost for the mac and cheese.

The oven safe ceramic dish that I had the chicken on, kept the chicken warm while I finished the Mac.

I do have a small DeLonghi (similar to the Breville) countertop oven in my pantry which I do use occasionally.