i have a very good idea how the double exposure be create represent mary pickford must have be photograph with her body cover in a black body stock for both of her role and and then two double one of which may have be mary sister charlotte be photograph in costume . then the footage of mary head be composited into the scene on the body of both double . no wonder it take hour to photograph the double exposure represent the optical printer a we know it today have not yet be invent and and it have to be do by hand and which take long than it would have do today with a digital compositor . therefore and it would have take a full working day to get the scene shot and to get them to composite properly together .