An insurer may not agree with any other insurer to adhere to or use any rate or supplementary rate information. The fact that an insurer adheres to or uses such material is not sufficient in itself to support a finding that an agreement to adhere or use exists, but such fact may supplement other evidence of such agreement. Two (2) or more insurers having common ownership or operating in this state under common management or control may act in concert between or among themselves in the same manner as if they constitute a single insurer.