Surface inward offset

[syachcho (s)]

I’m struggling to offset 2 simple planes. I want an ‘inward’ offset. Does someone know how to create an inward offset without choosing the surfaces as separate parts.

surfaces offset.gh (8.6 KB)

[inno]

if the surfaces are parallel and have the same normal, then one has to be offset a positive value, while the other has to be offset a negative value