I have an old friend who was shooting muzzle loaders back when they were in common use(almost). He believes they were intended to be used "back trigger first". However, it seems to me that the original MLs I've seen are usually most worn thin at the right muzzle. This says to me the right barrel was usually fired first and therefore reloaded more often. IME the gun recoils somewhat away from the cheek with the right barrel and into the cheek with the left. This is the conventional wisdom for firing the right barrel first and it's wise enough for me. This is for right handers and flushed birds, of course. AS others said, it's no trick to fire the left first, if you want to. That's why us two-trigger keep saying the double trigger gun gives us the most instant selection possible.
