Each car has a maximum gross towing weight.
If, on a weighbridge, the weight of your Leon, BMW and trailer exceeds that, then you are overloading.
If the combination "looks sensible" then you are unlikely to ever be caught.
If a policeman see you and thinks "small car towing a trailer and car, I bet he is over the weight limit" the chances are that you will be escorted to a weigh bridge where the weight will be checked. If you are over, you will be fined and get points, just like any other motoring offence.