You can certainly book the car for longer than the flights.

​​​But...

The revised T&C's on the double TP offer state that the car or hotel must be within the flight dates. This was to stop people booking a "holiday" with 5 days car hire at some other cheap time or even location.

Will you still get the double points if you book the car for longer. My guess is yes, but if they refuse you don't really have a leg to stand on.