Someone has told me it is a legal requirement to have a towing mirror on both sides of the towing vehicle when driving on the continent. It was news to me. Can any one confim this is the case as I can't find a written reference anywhere.
You have to be able to see past the caravan on both sides. If the car is wide enough to fulfil this requirement with the standard mirrors you don't need any. If not and assuming car and caravan are symmetrical about the centre line, you need two.
ps: The above applies if the inside rear view mirror is obscured so as not to allow one to see a 20 metre wide field of view at road surface height, 60 metres behind the eye point of the driver.