What version number of phpdocx are you using?
Advanced and Premium licenses include splitDocx to split documents, but this method only works with sections, and you are trying to split contents wrapped by specific contents.
phpdocx 10 Premium added importContents that allows importing contents from existing documents (in the main body content). Using this method you can generate new documents importing the exact contents in the main document body from external DOCX, for example creating a new document with all contents from CODE : LP01 (or START) to END, and other new document with all contents from CODE : LP02 (or START) to END.
If you are using a previous version than phpdocx 10, we recommend you to upgrade your license to be able to use the importContents method.