Horses that are trained properly can be VERY versatile. That's one of my favorite things to see and its something I strive for in my own training program. I don't find it hard to believe at all that a horse could do that, but I would want to see it for myself before I'd just buy into it. Actions speak louder than words and all that..
Also, it does depend on what level of showing you're talking about.. I mean, sure a horse could easily clean the top of both areas in smaller shows, or perhaps he has a stronger side and consistently wins there with more competition, but will still win with less competition in the other event.
Horses can learn their jobs very well.. And some have more than one job.