The Sage's Word on the Subject
KarinsDad said:
Quite frankly, unless someone comes up with an actual rule somewhere on it, this is a house rule extrapolated from other similar rules.
Oh, ye of little faith
Well, lacking a FAQ or Errata, I e-mailed the Sage. Got an answer on the same day, even. The results are abundantly clear on one point, not quite so clear on another....
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My question:
The Magic Weapon spell (PHB, pg 225) states, "Magic weapon gives a weapon a +1 enhancement bonus to attack and damage rolls." When cast upon a double weapon, such as a quarterstaff, does this apply to both heads of the weapon, or only to one?
The Sage's Answer:
Only one end.
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Which is quite clear. For purposes of the Magic Weapon spell, a double weapon is not "a weapon". It is two separate weapons. This is consistent with crafting masterwork and magic weapons, which treat the ends separately.
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My other question:
Similarly, does the spell Shillelagh (PHB pg 251) turn a quarterstaff into a single-headed weapon that does 1d10+1 damage, a double weapon that does 1d10+1/1d6, or a double weapon that does 1d10+1/1d10+1?
The Sage's Answer:
Also only one end.
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Which is not quite so clear. At least, not so clear for we who tear wordings apart with a nitpicker's glee

It certainly eliminates the d10+1/d10+1 case. However, since a one-headed weapon has "only one end", and a d10+1/d6 weapon has the spell affect "only one end", we still have room to argue over it.
I think, given the context (the word "also" strongly implying the second answer is much like the first) that the Sage means that the Shillelagh spell works like everything else that deals with double weapons - each head is treated as a separate weapon. This again would be consistent with other rules.
Do we need any more proof, folks?