You could consider growing a 2nd head - this would definitely justify having 2 helmets.
Otherwise, I have four. I keep one in each car, cause if I travel anywhere with the bike, the one thing I am SURE to forget is a helmet. My wife makes me buy one if I forget, which is why I have four. One is a spare/loaner.
But, I tend to go to a basic inexpensive helmet which meets the current safety standards.
There is an interesting long article on helmets in the current Bicycling magazine.
From the article:
"No matter what, it's got you covered, right? Maybe not. Turns out, helmets were developed only to protect against massive head trauma, like cracking open your skull, and simply haven't been designed to prevent less immediately catastrophic injuries like concussions. What's more, none of us—not you, not me, not the helmet manufacturers or even the testing agencies—know for certain whether your helmet will prevent you from getting hurt."
http://www.bicycling.com/training-nu...ous-protection