To me that's the passengers job. They need to wave so that the rider can keep both hands on the bars. Most folks riding two up that wave back, it's the passenger who has waved back. Makes sense to me but i'm simple minded

waving is respectful acknowledgement of another rider and, unless two hands are absolutely needed at the time due a return, but if you need both hands at the time a nod useually works well, in the deep south at least !

Over100, you got that right brother! I wave to all ... all HOG riders wave around here....yet many of the sportriders don't , maybe they feel intimidated by my 'thunder'!!!