you situation might ne a little different than mine, but I introduced 4 pups to gunfire by bringing my older dog along,... they see his excited reaction to the shot, and follow along,
on a side note, I do everything as if i'm introducing both dogs to gunfire for the first time,... I just makes it a bit easer when they have an older dog to learn from.
this is MY experience though,.... your situation might be a little different