A Non-Primary Payer, that is not flagged as a payer will only be able to see Lease Docs and other uploaded attachments. They will not be able to see any payment related information, nor will they be able to see any payer statements.
A Non-Primary Payer, that is also flagged as payer has the same access as regular contact with additional payment information access, like the Primary Payer. This includes the ability to see Charges, Payer Statements, and to set up Bank account information. Typically this is a backup payer, however, they still can make payments from the portal.