"I currently have a combo stove/microwave unit"

Do you mean that you have a double wall oven whose upper oven is a microwave and that you're considering replacing that with a single wall oven and a separate, built-in microwave oven?

"some minor rebuilding of the cabinet"

How did you conclude that replacing the double wall oven with separate appliances would only require "minor rebuilding"? Did you check the dimensions of the double oven and check the dimensions and installation manuals of the new appliances?

I believe tapping off of a range circuit is allowed for such as clocks. I can't recall the exact wording. I would bet a donut or two that the microwave oven installation manual calls for a separate circuit. You could do as below:

In general, a 240 Vac circuit to be distributed should go to a branch breaker box where it can be allocated as desired using properly sized breakers for each subsidiary circuit. Of course, this assumes in this case that there is enough current capacity in the original feed that it can supply more than just the replacement stove. Since determining the actual allowed ampacity of various circuits requires study of the National Electrical Code, this is most easily determined or accomplished with the help of a licensed electrician.

Minardi, I expected as much. The only way I could see it reasonably working (from a code standpoint) would be to run the same gauge wire from 240V circuit to new 120V box so the breaker matches the full wiring.

cruise, correct, a microwave above an oven as a combo unit. I concluded it would be minor as the cabinet has significant extra space height-/depth-wise, fits a standard 30" oven, and my woodworking skills are more than adequate to build a fully custom kitchen cabinet set, let alone add in some internal rails and a new mid-rise floor for a new microwave.

But two appliances cost substantially more:

• Frigidaire FCWM3027AS Electric Wall Oven with Built-In Microwave (CR top-rated; $2300)
• Cafe CTS70DP2NS1 Stainless Smart Single Wall (CR smart buy; $2663) plus matching Cafe CWB713P2VS1 built-in microwave ($2158)
• LG WSEP4727F Smart Single Electric Wall Oven (CR smart buy; $1700) plus LG MZBZ1715S Stainless Steel Built-In Microwave ($2300)

While a multiwire branch circuiit is a perfectly safe, legal, and common means of distributing 240 and 120 V outlets from a 240V feed in a structure other requirements will preclude it in this case. I believe a range must have a dedicated circuit, similarly, though a microwave can share a circuit, the power demand of most units precludes it. In practice a dedicated circuit for the microwave is a much better idea. For combo units you need to follow the manufacturer's instructions.

cruise, of course two separate items cost more... initially, and if you cherry pick the units for comparison. I mean, seriously? You chose a $2,200 microwave as an example?

Right now I wouldn't be trying to replace a working microwave simply because my oven broke, so I could install a perfectly-serviceable $1k oven. That alone would make separate units worthwhile. Then add to the fact that I already have a high-end convection microwave elsewhere in the kitchen, so it's not a separate product I would have to purchase. The real cherry on top, I have significantly more options when purchasing separate compared to what companies force me to choose from with combo units.

"You chose a $2,200 microwave as an example?"

Yes, to match the Cafe oven that would be installed below it. It was the cheapest matching microwave.

"I could install a perfectly-serviceable $1k oven."

Sure, the Whirlpool WMT55511KS is only $800 but I assumed you'd want to match the microwave to the oven which I may not be the case. And I chose lower-priced, CR-recommended ovens.

"I have significantly more options when purchasing separate..."

True if you don't care whether the oven and microwave match. There are lots of single wall ovens available but significantly fewer built-in microwaves.

"I already have a high-end convection microwave elsewhere in the kitchen"

Now that's confusing.