I have read on the internet that the rear trigger can be fired with the middle finger.
With a decent sxs I have to ask why it would matter. It's way more natural to pull the front trigger first. I've had a sxs with an extremely light back trigger, which should be shot back trigger first until I get it fixed, however I don't see any need otherwise.
Choke is the reason why it matters... In a decent sxs and o/u you get to pick your choke. Usually right barrel first (more open choke) left barrel tighter choke. Usually your first shot is at closer distances. Or if the game flushes further out you can pick your tighter choke.



























