actually, the syntax is simply addweapon and addmagazine. so for example, an Ah-64 named
chopper could be given another machine gun by putting the following in the init file (or elsewhere) -
chopper addmagazine "ZsuCannon"
chopper addweapon "ZsuCannon"
you should always add the magazine first to make sure that the weapon is loaded to begin with...
EDIT - just tried the tank shell and the a10, doesn't work. many combinations do work however, so experiment