I think it's the two added up, makes sence. If you have two lucky characters you have twice the chance of getting a really good item than if you only have one lucky character.
But don't forget, if you invested alot in luck, you might get awesome stuff, but your characters remain weak. It might even turn out that they're too weak to use the weapons or armour they find <img src="/ubbthreads/images/graemlins/winkwink.gif" alt="" />
Tovi 