There is no way to know without trying it. And even then there is no way to know without really riding it, and by then it's not returnable. But such is saddle fitting and owning horses.
I will say that the better the saddle quality-wise, the more chance you have of it being versatile. At least that's been my experience.
I have ridden saddles with FQHB on my horses and had them be too wide, but then the saddle I have now that is supposed to be FQHB and it fits BOTH of my horses, who are built very different, and I have ridden them ALOT in it. Like 5 hour trail rides with a lot of cantering and hills. So if it were going to sore one of them it should have done it (and I weigh around 200 lbs.).
One is a wide Mustang (like an old fashioned QH) and one is a Missouri Foxtrotter. While the Fox Trotter is not narrow, she is definitely not built like a QH either (gaited horses tend to have more of an "A" shape to their withers).
I guess what I am saying is, yes, there is a shot it could do a good job of fitting them both, or it may not. But you never really know until you try it. And even if it looks like a good fit, you never really know until you put some mileage on it. I have probably gone through 15 western saddles in as many years.
It just always seems like I am looking for my dream saddle!
By the way, I am thinking of getting a Billy Cook too. Which model are you looking at?