Here is a neat trick that lets you re-program one key fob to unlock all your cars. It is like a universal remote, but for your car.
The idea of this is to reprogram it to recognize a fob so that it can be paired with four remotes and also to create duplicate fobs for a single car. The steps are slightly different with the model of the car, so be sure to follow the right one for your car.
We had imagined the best scenario for this is if you park your cars in a garage. Because you might end up leaving one car unlocked while driving the other away.