How is health insurance primacy determined?

I currently have I two group coverages:

One BCBS with birth month August
One FEP BCBS with birth month of October

My claims are being denied/held as both are stating the other insurance is primary.

Blue Cross and Blue Shield of Montana, a Division of Health Care Service Corporation,
a Mutual Legal Reserve Company, an Independent Licensee of the Blue Cross and Blue Shield Association

Telligent is an operating division of Verint Americas, Inc., an independent company that provides and hosts an online community platform for blogging and access to social media for Blue Cross and Blue Shield of Montana.

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