Okay. Playing Devil's Advocate here...
My main problem with the Two-gun Mojo stunt is that it makes a mockery of Spray attacks. Targeting two targets? Don't bother splitting your attack in half, simple subtract one from each result!
At lower-levels of the Gun skill rating, the differences aren't exactly high and are reasonable enough for the benefits of a Stunt.
A Fair (+2) Gunman attacking two targets, splits the net roll [keeping it basic] of 2 by 2, for a total of 1 against each target.
Using the Stunt, it'd still equal an average attack against each.
A Great (+4) Gunman does the same, for a total of 2 to each attack. Each target receives a Fair attack.
Using the Stunt, this becomes a Good attack. Now a difference creeps in between the two attacks.
For every two you increase the Gunman's skill, the gap opens up by another one. There will also be a noticeable different when applying manoeuvres etc, as they won't be split either (but I guess, would only apply to one target; unless fate points are spent). That is assuming they split the attack results evenly. Now. This in itself is not too-unbalanced. However the ability to attack a single target twice in one round simply seems over-powered, especially at only a -1 penalty.
Usually to make 2+ attacks, you have the actually split the result. Even the ability to make multiple attacks against one target using the Spray attack rules seems more balanced; and that is probably a better suggestion then my last one.
Perhaps limiting your current stunt to the same &/or adjacent zone would help. I can't see someone wielding two pistols being very accurate at a 3+ zones distance. Which would give it drawbacks, and a character focused on Sniping would be as balanced...
Just my thoughts. I'm not trying to discourage you from making new and inventive stunts, I just try to stay on the side of caution when it comes to balancing issues. In fact, I'd rather encourage such ideas, and the discussions about them.