Because of how harm works in Apocalypse World, there's actually two related but different questions here:
How many times do you get shot?
This depends solely on the narrative. The GM could decide they both shoot at you, or just one shoots at you, and even how many times they shoot at you. One of the descriptions for 2-harm in the book (AW second edition book, page 204) is "a guy with a hammer, a crossbow, a handgun, a smg" - so any amount of shots appropriately fired from a pistol counts. Could be one nasty hit and two or three scratches from a triggerhappy pair of gunslingers. Serious, but not outright fatal.
As you note in the question, harm received does not grow linearly with the amount of shots taken in Apocalypse World - gangs and automatics would be quite deadly if that were the case.
Which leads us to the related question...
How much harm do you take?
This ties in with gangs.
Basically, when a gang fights a gang, they exchange harm as if they were two individual people fighting, modified by their size difference. The smallest "proper" category of a gang is "small", 10-25 people (page 211), and if they had 2-harm weapons, they would inflict 3-harm upon you. It would make no sense for two people to be more deadly than twenty, so the reasonable course of action is for two people, in situations where they're effectively a single threat (eg. shooting at the same target), to inflict 2-harm. Also note that, on page 242, "a guy or two" is explicitly listed as a gang size category - so you can consider the situation is harm-wise equivalent to two same sized gangs fighting.
Armor and move effects are applied, likewise, per instance of harm received, not per shot, so you end up with taking 1-harm (2-harm minus the 1-harm reduction from Seize by Force).