Reporting on Primary Custom Object 2 record related to Opportunity

Hi -

Custom Object 2 has a many-to-many relationship to Opportunity. In the Opportunity record and in lists and data exports we can see the "Primary" Custom Object 2 record (Name, External Unique ID, etc). Is there a way to report on this in reporting and analytics?

i.e., we want to have a report that includes Opportunity and Custom Object 2 information, with either a flag on the CO2 record that is primary or a way to filter such that only the primary CO2 record is displayed in the report.

I can't see a "primary" field or way that this is indicated in the reporting/analytics subject areas.