At least in places where you get the contract at the exit booth, won't you need to have your license in the car? Otherwise some random woman is trying to take out a car that is in your name. Maybe (?) you get away with it if same last name, but would not count on it. Or maybe you drive one car out, walk back in, and drive the other out?

Why not just put the second car in her name? Hertz points? Status / gold service?

You are also likely to get an email from Hertz saying "You have 2 reservations same time / same place. Pick the one you want to keep or we will decide for you". But there is an option to "keep all the reservations". Happened to me yesterday (I am holding several reservations for different sized cars and return dates, as plans are not final, and I sure didn't want to wait to make the reservation, with what rates are up to these days!)