No it will not. Source: Personal Experience with 2 Gible Fangs.
Yes, it does. I saw someone in Sky Stairway on Youtube and they had Flamethrower +6 when they used it on Rayquaza (How dumb is that?), I think. (They were probably cheating :P)
Since you were wondering about stacking in this game, I got my question answered about multiple Friend Bows/Golden Masks/Amber Tears held by different party members. Guess what? Those stack too! :D
CORRECTION: These last items DO NOT stack. Sorry about that.