Andrea had this to say:
Jamming twin guns (A firing planes)
When you fire with an A weapon, that represents twin guns, you could jam one of the machineguns or, more rarely, both of them.
If you are at short distance, you jam a machinegun if target picks up a jammed Damage card, you jam both of them if he picks up two of them. Firing at long distance, you jam both of them if the target picks the 0 red jammed Damage card, only one if he picks the 2 red or a green jammed Damage card.