Why can I not park my second car in the visitor parking spaces?

Your lease specifically prevents you from doing this, stating that the Visitor Parking spaces are only for the use of visitors.

You may be able to rent an unused garage or space from another leaseholder or tenant. You can ask the Estate Manager or the Concierge if they know of anyone who has such a space to rent. If you cannot find such a space for your second car, it will need to be parked outside the Estate.