The back trigger is the set trigger. You pull it until it clicks which indicates the front trigger is set. Any pull beyond this point doesn't do anything. You might want to take the trigger assembly out and clean it well with solvent, then see if there is a bind somewhere. Oil it again with Breakfree or similar.
You may want to get a new screw for adjusting the set trigger. The one which comes with it is too short. I don't remember what size it is, but it is a metric thread. Take it (the screw) to th hardware store and see if they can get you another one 1/8" longer. Otherwise it has very little effect on the let off of the trigger.
I purely love my .54 GPR flinter. I have modified it by removing the butt plate and toe plate, straightening the butt plate out a bit and cutting the corresponding amount of wood off the toe and shortening the toe plate. I had to do some filing on the butt plate and rasping of the wood also, if I recall. However it no longer bites me under the shoulder. At the same time I sanded the stock down and removed that gosh-awful dark stain they put on it, then refinished it with Lin-Speed. I also coned the muzzle so I can thumb start the balls. I'm thinking of maybe putting on a patchbox this winter.
-Kees-