Using multiple wands at once?

[rainqueen01]

I'm reading DH again, and came across something that made me curious. When they're in Malfoy Manor, fighting to escape, Harry stuns Greyback, while holding three wands. It sounded like the "triple spell" that hits Greyback was more powerful than it normally would have been. If one person's spells using more than one wand are extra powerful, then why doesn't everyone use as many wands as they can carry? I'd think you wouldn't want to do that for every day stuff where precision is just as important as power, but when you're fighting for your life and need the strongest defense and attacks you can get, why wouldn't you?

Or I wonder if this only helps when they're borrowed wands that don't work as well, and the best spellcasting is from one wand that chose the wizard?
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[ResidentWino]

Wands seem to be treated like a controlled substance with the wandmaker acting as both prescribing physician and pharmacist. When someone loses or breaks a wand ... Hagrid, for example ... they can't just go and get another one unless they are supposed to, (like Ron or Neville).

Otherwise, I would expect some wizards to have multiple wands. Lockhardt would have dozens of Swatch wands in different colors, matching his robes.
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