You scratch my back, I'll scratch yours

Sassy was originally purchased as a horse for Adam's wife.  He'd ride Tude and the wife would ride Sassy.  Eventually Adam got rid of the wife, but kept the horse.  If he leaves for a few days  of riding, he takes both horses and takes turns riding them.  Sassy, a half-Arabian, gives him the most comfortable ride of the two.  Tude, however, is his first and favorite.  
The two horses are best buddies.  If they happen to get separated, they'll whinny and run the fence until they're back together.  They do things for one another, as best friends do.  For instance:

Yep.  They'll simultaneously scratch one another's itches.