I guess it is possible, but it is human error; nothing else. I acquired certifications for 25+ sales people and finance managers at a dealership that sold 4 different manufacturer's lineups. It is possible to sync those keyfobs to two vehicles, as the keyfob itself is the actual authenticator to unlock the vehicle, in the communication between car and keyfob; and then car just authenticates that, "yes, you have sync'd me to this key before." Unlocking two cars with the same keyfob, regardless of whether or not it is a proximity fob with a continuous signal or a regular old push-button-to-unlock-fob, is only a matter of sync'ing both cars to that fob. It just means at some point in time, there was a cruddy mechanic who didn't decide to wipe the key because, "woops, I just sync'd this key to the wrong car... I wonder what I need to do." They leave the car to go ask someone, and then discover the key is still opening the car it belongs to. Works for them.
Those keys didn't come from the OEM ready to open both cars. No way, no how.