Umbran said:
One doesn't really need other sources. A strict reading of the PHB will suffice (where bold face is my own emphasis).
PHB, pg 225: "Magic Weapon gives a weapon a +1 enhancement bonus to attack and damage rolls."
PHB pg 125: "You can use a double weapon to make an extra attack as if you were fighting with two weapons."
Note the words
as if in this sentence. The fact is, a quarterstaff is one weapon, either end of which can be used to attack. It is
a weapon (as you pointed out in the Magic Weapon description), not two. If it gets Sundered, the entire weapon is sundered, not just one end.
Also, two weapon fighting is a fighting style, not a weapon type. You can use this style with either two single weapons, or one double weapon. One. Plus, you do not have to fight with this style with a double weapon.
Umbran said:
"Two weapons" != "a weapon". When using a double weapon this way, the combatant is really using two weapons. But the spell only affects one, so only one end can get the enhancement. QED. The rules for crafting masterwork and magical weapons are only further examples of this general idea - a "double weapon" is actually two weapons who just happen to share a length of handle.
What you are doing here is extrapolating a set of rules which are there merely for balance in creating superior weapons, to other areas. The
rules do not state that casting Magic Weapon does not affect both ends of a double weapon, rather that is an extrapolation of other rules. The rules say that Magic Weapon "gives a weapon...". A quarterstaff is a weapon, regardless of which fighting style it is used with.
Sundering or disarming is sufficient to illustrate this.
Quite frankly, unless someone comes up with an actual rule somewhere on it, this is a house rule extrapolated from other similar rules.