If I have a joint loan, collateral (vehicle) is owned as

Minnie or Mickey Mouse

and loan is in default, Minnie and Mickey are now divorced, Minnie comes and drops off keys, vehicle, and title and signs a surrender form (I'm in WI if that helps) does Mickey also need to sign or can just one of them? I will have her sign the title as well, but just wonder if anyone has any history with something like this. Thanks for any advise you can offer.