Thanks for the image, I think I get what you’re trying to do. Check out the enclosed pictures, of a kitchen I just completed with a similar situation with the pantry cabinet along the back wall. It had drawers at the bottom, then two larger bays on top. You’re correct in that there’s not a built in, elegant way of doing that yet with CabWriter, but you’ll be happy to hear that Joe is hard at work right now on a solution, but it’s not ready yet.
In the meantime, you can do what I do in this situation. Draw a short base cabinet with a drawer at 24″ high in your case. Then, draw an upper directly on top of it and overlapping it as necessary to make things line up. In my case, there was a 1 1/2″ rail separating the bays, so I made the lower cabinet have a 1 1/2″ top rail and the upper cabinet had a 1 1/2″ lower rail. Then, the starting elevation of the upper cabinet is the lower edge of the top rail on the lower cabinet, if that makes sense. In the end, you end up with two cabinets; the upper rail on the lower cabinet and the lower rail on the upper cabinet occupy the same space. Then, you delete one of the rails and the stiles on one of the cabinets and extend the remaining stiles to cover the entire cabinet. You’ll have to stitch up and side panels or sheets as well as the backs, but it doesn’t take very long. You end up with one cabinet that has parts with different cabinet numbers, but it’s workable. If it’s a frameless cabinet, it’s even easier because you don’t have to stitch the stiles and rails together, but you may have to deal with a side panel and the backs.
Hopefully, we’ll have a more elegant way of doing this soon, but in the meantime, give it a shot and see if you can get it to work and let me know if you need more help.