I came to this site while searching "double dinking". I grew up in Hobart and can assure readers the term double dinking was in common usage while I was growing up in the 1970's and 1980's. I can't remember the legalities involved but we all did it. Here in Japan it is common in Kagoshima, I can't speak for other parts of the country. The locals call it ふたりのりwhich would translate 2 people riding. Boys "straddle the saddle" and girls typically sit "side saddle" while courting.