Untrue about both guns jamming or neither. I've read combat reports from Frank Luke (American ace, flew SPAD XIII's) referring to continuing to fight after one gun jammed ("experienced a stoppage"), and shooting down an enemy with the remaining gun. Also, there are photos of WWI cockpits clearly showing two separate triggers on the joystick for firing the MG's, cleverly placed so that the pilot could fire one or both guns at his choice. I'm referring to many easily available photos of the Albatros DIII and DV cockpits, with twin Spandaus, not just the RAF SE5's.
In my "House Rules", here's how I handle ammo and gun jams:
AMMO and JAMS
Use only the "B" damage deck for aircraft machine guns. Deal each player 10 cards PER GUN, face down, representing their available ammo. When firing more than one gun, use one "B" damage card or EACH gun. For close range shots, use a second "B" card from the undealt pack for each gun.
OPTIONAL: Trigger-happy pilots with enough ammo to burn may, if they wish, double the number of damage cards given to a target. They risk increased jams and running out of ammo sooner, however.
c. GUN JAMS
All guns are jammed separately. Each jammed gun must be unjammed separately. Guns cannot be unjammed if the other gun is fired in the turn. If the firing player has "Bullet Checker" skills, ignore the jams resulting from damage cards with the GREEN "X".
d. LEWIS GUNS
Lewis guns (and some others, such as the Schwarzlose and Hotchkiss) have limited ammunition. After inflicting 5 cards of damage, treat the Lewis gun as "jammed" whether a jam result is shown on the damage card or not. This represents the pilot reloading the ammunition canister. Observer guns, whether Lewis or Parabellum or whatever, receive only one "jam" marker; since observers don't have to fly the plane, reloading is a simple task. You MUST clear any jams BEFORE attempting to reload. Pilots can only reload Lewis guns while flying straight and level, not firing any other guns and not dealing with any other "special damage".