This is a residual effect from other games. You don't zoom that to this micromanagement level. Just the same as we don't count bullets, we don't count the numbers of guns they decide to shoot off in any one roll. Doesn't matter if it was a 100 rounds holdings off those baddies or 200. Deciding to fire with two guns isn't any more effective them firing with one. It's just cooler for some people. Just like you don't do round by round combat, you do action by action combat, which could very well cover hours of fighting in two rolls, or seconds, depending on the stakes and the scale.
To sum up, No. This custom move lacks any stakes, any doesn't provide anything the game doesn't already do. This is a bad idea.
If someone is firing two pistols like some action hero... wait they are an action hero. The answer is okay, cool! On a hit, describe them shooting people with both guns.