You can do it at the Kiosk. you will need to have the second driver with you when you exit the garage. The exit agent will need to see both licenses and credit cards.
I am not sure you can add the second driver with online check in, but, if it is allowed, the 2nd driver will have to be present when you leave the garage and will need to present his/her driver's license and a credit card in their name. (additional drivers need to present the same "stuff" as the renter).