Seems like its possible to do, but will require full access to sources, since writing test methods on forum and waiting for response will be extremely inefficient work.
I would start in doAttacking method where
Item* tool = getWeapon();
const Weapon* weapon = g_weapons->getWeapon(tool);
it takes the single weapon
and then:
weapon->useWeapon(this, tool, attackedCreature);
It executes the attack (described method is in weapon class)
There would be need some new function in player class that would take both weapons and then new weapon-function that would calculate damage from them.
imo you need to do it by yourself or make request/job, giving someone full access to edit,debug and tests sources.