How to get a subsurface?

I made a planar surface with triangle shape and want to get 2 subsurfaces: a trapezoid and a triangle
so I define 0~0.5, 0.5~1 in U and 0~1 in V and try to use Isotrim to get 2 subsurfaces.

But Isotrim creates 2 rectangulars which is not want I want.

How to solve the problem?
Rhino and GH files are (23.4 KB)

Planar surface code shrinks from a quad plane. The quick answer was to do an isotrim from an edge surface. A more complex answer with broader use would be to isolate an isocurve, define subcurves, and then create a surface from those, and also make sure your U and V domains are the directions you want them.
For more complex shapes, untrim the surface, isolate the domain, and then retrim to match the original surface boundaries. SubSurface Isotrimmed Edge (13.4 KB)