It shall be possible to add a relationship template (connect two VFC instances in the composition view, and select 'Add Operation' at the 'Operations' stage of defining the relationship), then export the service. Tosca generated is wrong format for implementation. 

Expected format: Implementation: <primary_artifact_name>

Generated format: 

toscaPresentation:
                  duplicated: false
                  artifactVersion: '0'
                  serviceApi: false
                  isFromCsar: false
                  generated: false
                  name:  <primary_artifact_name>
                  mandatory: false