Usually credit lines are issued on the equity in the home.
They then become secondary mortgages to your primary mortgage.
If there are multiple owners of the property, all the owners would have to complete the application.
If you have one of the few 1st mortgages that give credit line options on that mortgage, you would need to contact your lender to confirm their guidelines on whether only one of the borrowers or both of the borrowers need to sign for the extended line.
Best of luck