The more interesting part of this discussion is if "Bob"'s spear was a +1 Defending, but he attacked twice with his Longsword...would he be able to assign those 2 points of enhancement bonus to his AC?

This further dovetails into ridiculous stacking of Warning, Eager, and other various passive weapon enhancements on Armor Spikes, Shield Spikes, Boot knives, hair knives, mouthpick weapons and all the other crazy options people toss out.