The Participants are is likely to only show some default roles, like editors and authors - not custom ones. Have you tried assigning the 2nd author as a participant (using the Assign button adjacent to the Participants). You may have to do this manually, going forward - typically OJS only has one author (the one doing the submission) as a participant - all others have to be added manually as participants.
The contributors list is used for metadata (i.e. “who wrote the article when it’s cited somewhere”), and the participants list is used for workflow access (i.e. “which user accounts have the ability to work on the submission”). When a submission is first created by an author, that author is automatically added to both places, but there is no connection after that; if you add a second author under Contributors, they won’t by default be added to Participants. (And it should be that way – generally only one author is responsible for working the submission through the workflow.)
Regards,
Alec Smecher
Public Knowledge Project Team