Well this isn't really Dual Weilding but it is holding 2 weapons. kind of.
A good modeler make a needler model (fp) which looks like 2 needlers, then (if this works) open needler mod2.xml or needler.weap.xml and change the vertices to something different, like a warthog (so it should be able to have a model as big as a warthog) then change whare the projectiles come out (says something about it in scripting and Programming) and insert back into halo by rebuilding.
