See also
Hitchhiker. A means of travel using spare passenger capacity of vehicles heading in the same direction, usually for free.
Jack Kerouac found out in
Big Sur that although truckers might take on hitchhikers, Mom-Dad-and-the-kids on vacation in their packed station wagon certainly won't.
One possible origin for this term refers to two people sharing a horse on a journey. If only one person can ride the horse at one time, that person rides a quarter mile or half mile along the road and hitches the horse. They then hike. The second person eventually hikes to where the horse is tied, saddles up, and rides the next quarter mile, passing the first person on the way, then hitches and hikes again. In the end, each person has ridden the horse half the distance and walked the other half.
I would think most horses could take two riders together in a pinch, but I'm not a horse person. I tried this method with a bicycle once, though, and it seemed to have some advantage.