Using Revit natively, you’d have to create the topo like that and then do a Split Surface and then delete the middle one. I don’t think Dynamo can get around that limitation, and I don’t think the Split Surface tool is exposed in the API. You can vote for it here: https://forums.autodesk.com/t5/revit-ideas/split-topography-method/idi-p/7501354
Yes I believe that would yield a direct shape or freeform object which is a bit different to a topography. You could always create subregions after using Dynamo by making the topography and then collecting any loops in the base sketch which are smaller in area than the outer loop. Usually I’d do this computationally by getting naked mesh edges, flattening them down to the XY plane and grouping them into closed curves, measuring their area, sorting them by that and then using List.Deconstruct to take the biggest/rest.