Often a complex surface is very difficult to be trimmed by anything other than its own isocurve.
In the attached file I have created the curve in Rhino with the split by isocurve command.
I tried everything to try to split that surface with a curve created inside MOI (not using the curve created in Rhino), but everything failed including trying to create a new curve with the on surface snap on.
Is it possible to create such a curve inside MOI?