Search terms: "two riders on a horse," "riding double," the little_details "horses" tag. I checked Judith Tarr's book Writing Horses too, but didn't find much. A lot of what I have found deals with unconscious/injured people, so it doesn't exactly apply.
So I have two characters who've managed to render themselves extremely inconvenient to some nasty powers-that-be, and need to run for the border before a group of antagonists catch up and kill them. Between them, they have one horse and maybe half an hour of lead time. Obviously they are not going to be able to outrun their pursuers - also mounted - but in the interest of getting the biggest headstart that they can, how fast and far could a horse travel with two riders? We're talking two relatively big guys here, not wearing full plate armor but definitely weighed down with gambesons and chainmail and weaponry, and while the horse is no delicate little racehorse it's not a giant draft horse either.
They are fully aware that at some point they are going to have to get off and walk (the plan is to cut across country, through terrain where their pursuers will have a hard time following on horseback). But they are hoping to get as far away from the danger zone as possible first. Can any horse people around here help me with this one?