Or to be more precise:
The purpose of the cloud-cars (or twin-pod-cars ... don't know?) is to protect Cloud City isn't it? If they turn their back on an approaching, unknown vessel they are not serving their purpose, mission or "job" as they could be blown out of the sky and Cloud City could be attacked.
Their mission: Protect Cloud City, identify unknown vessels, guard the approach of unknown vessels, escort unknown vessels to their landing platforms. They have no reason to trust unknown vessels. So they should fly at each side of the Falcon or behind the Falcon if there are only two of them. If there are three of them one can fly in front of the Falcon while the other two are flying beside or behind the Falcon.
The scene can be "repaired" if a third cloud car is added guarding the Falcon from behind. As I recall there will be no continuity error if there are three of them. It would also add to Leia's sceptical expression when they arrive at the platform.