Thank you, Tom.
I'm an illustrator with no architectural background. I'm a bit surprised that it's not possible to create a mid point in a typical wall (or use an existing midpoint) and drag it upward. I have to do a similar procedure in Vectorworks Architect in conjunction with a roof element.
I'm wondering if this feature is something that could be added to a future version. It is definitely easier (and more direct) and doesn't require the assistance of another element such as a roof.