You can generally do this, but make sure you have a copy of your voucher with you. If on the Gold option, you would have to return the first car as empty as possible of gas, then the second car collect full and return full. If on the Silver option then return both cars full (to avoid 2 refueling charges).

Note that Alamo are only required to give you the class of car you want, or better (not an exact make and model) so if you go to another depot part way through, they do have a right not to let you swap the car, but I know from experience that they are generally very accommodating so it should be fine.