This is typically due to the merge settings on the account, or because an existing contact record/supporter id was used, instead of creating a new record.
It is best to register the second record with non-matching criteria (if you have Real Time Dedupe enabled. This will prevent the records from being merged automatically). The system needs 1 Supporter ID per role. As a result, you should have 2 records, each with different Supporter IDs that relate to different roles.
For future registrations, we suggest registering the secondary record through the back end with non matching information, to create the second supporter ID. Then, you can promote the record to team captain through Bookkeeping, if needed.