For a weapon in each hand, it would be quite easy - i just need to replace the weapon i connect to the animation. For two-handed weapons and melee weapons like sledgehammer a new animation could be made, using the weapon as a crutch. I'm not sure about the BigGuns, though and i'm open to suggestions in that matter.
Parts of the animations could be left as they are, just the new elements should be tweaked accordingly.
Btw, do you know how a animation can be mirrored?