It is very true some of these bikes may be moving a lot faster than your horse, BUT, On most trails the horse has the right-of-way over the bikes, SO, if you get your horses accustomed to the bike, then riding with them is not a problem, BUT, you may have to get out of there way, simply because of the speed they are traveling.
The only way I know of getting horses accustomed to bikes, is simple exposure. A bike coming up from behind sounds a lot like a snake about to strike. Find some bike riders that will work with you, and start out a little bit at a time, in a safe area. It takes time, but can be done.