You can not rent a car for someone else by simply adding them as a second driver and have them depart the lot without you checking out at the gate. Tried that. If the contract is under your name, you have to be in the car to present DL & Credit card.
You can however have two cars out at the same time no questions asked. Do that all the time.
Work around: Check out one car with you and driver #2 on board...once you are through the exit, drive around the front get out and get your car.