Thanks for all the advice everyone :)
Just so you know, the road is wide and cleared regularly so there is no ice or snow. Also it's usually fairly dry and the horses have never slipped on it. I'm actually thinking that it may work for the younger horse to trot for small increments up the road, and my older gelding can actually keep up walking fast. I've taught the younger horse to do a "little trot" and my old guy has such a reaching walk that they could probably go the same pace. I'd probably only be doing this once or twice a week, and of course I'll start out slow. Of course, it would be much easier to walk them both separately, but Scharm (the older horse) has a fit if he is left alone as we only have 2 horses. So thanks for the advice, and if it turns out that we only end up walking then I guess that will have to do. At least they won't just be sitting in a paddock.