Two machine guns on a fixed-wing aircraft or helicopter are impossible, I fear (and the possible ways include nasty collisions and/or weird looking behavior when those aircraft do any sort of roll or pitch change.)
But there's a workaround, if you're willing to play around with p3ds, config files and eventhandlers. You could, theoretically, use modified missiles instead of machine gun bullets:
First you'd have to modify the .p3d file - helicopters have sections for missiles - they use proxies. You'd have to place the proxy at the machine gun model (exactly where the bullet would come out) - then you'd have to edit some standard unguided missile config section. Remove the init time (else the bullet falls before it shoots off), set the thrust to a high value, set the thrust time to something under 0.5 seconds. Then of course, make a model for the bullet. And then, of course, a 'fired' eventhandler which executes a script which controls the "bullet's" speed. It's not a real machine gun, but rather a simulation of a machine gun. I tried things like that some time ago (with lasers for TIE Interceptors) and it pretty much worked.
I hope that was somewhat helpful.