If two owners own the car and their names are on the title and it says something such as Jane And Jack then both owners must sign the title to the vehicle to sell the vehicle.
If the title to the vehicle says Jane and/or Jack then just one owner needs to sign the title to the vehicle.
So if it says and/or on the title then only one of those owners has to sign the title and if it says just and between the names then both owners need to sign the title.