(NOTE: I believe I already know the answer to this, but here goes):
A friend of mine playing his first Feng Shui game took the guns schtick Both Guns Blazing and then took Signature Weapon. I told him that, as the schtick says, it applies to ONE specific weapon i.e. one of his Glocks but not both. After a brief discussion he erased Carnival of Carnage and took a second Signature Weapon schtick for the other gun. Did I do this correctly as per the rules?
I'd say so. Although there's no reason he couldn't use BGB with one signature and one non-signature weapon, besides the loss of three points of damage.
Yes, that's correct. You might want to make sure he's aware that BGB and Signature Weapon don't have any effect on unnamed GMCs, since all you care about for them is Outcome > 5. Extra damage from BGB or SW doesn't matter. For this reason, I like to offer some additional schticks for Signature Weapon/Path of the Selective Master:
Signature Weapon II: You take out unnamed GMCs on an outcome of 4 or better. You may buy this schtick one more time to take out unnamed GMCs on an outcome of 3 or better, but that's as good as it gets.
Signature Weapon III: This schtick can only be taken with a gun. Your signature weapon never malfunctions or jams.
He would do [damage for gun 1] + [damage for gun 2] + 3 for the signature weapon - [opponents toughness × 2] + Outcome.
The thing is, the first schtick is totally worthless. Doing two attacks gives -2 anyways and give double damage if both attacks hit, even if you didn't have the schtick. The only difference is that you roll once instead of twice, giving the possibility to boost that roll with fortune points.
In the rules as printed, you can't attack the same target twice in an action. You can attack X targets with -X AV, but you can't concentrate them on the same target. So the first schtick definitely has a use.