Multipart features are not supported for the Feature Edit tools in LP360 standalone and may result in an error message such as, “…Complex polygons is not supported”.
The polygon must be a single part feature. This can be verified in the Feature Analyst dialog:
Probable Resolution #1:
You will need to first Disassemble Multipart Feature, then run the Split Geometry, Double Line Drain or other such Feature Edit tool.
You can then assemble the parts back into a multipart feature using Assemble Multipart Feature.