I have always changed saddle pads when I use a specific saddle on 2 or more horses. But since I make my own saddle pads, I always have a good selection to choose from, so basically, each horse had his or her own set. But the same logic might well be applied to quarter sheets. They are not that expensive, especially if you can make your own, so I would suggest you seriously consider having one per horse. If you blanket during the winter, you block UV rays, which along with doing a good bit of damage, can be helpful in killing bad things in the horses coat. Also, when it is cold, we humans tend to be less meticulous in our grooming, and a bath with a shampoo containing anit-fungals is just not practical. And that way you can get the fit right for each horse (and each saddle).