The pun is in "two tents" sounding like "too tense", while the man is repeating two types of tent. The other way around the statement doesn't make sense, destroying the logic of the joke.
But it makes sense for him to say "you're too tense", too, as the man walking into the bar is obviously tense and stressed.
I'm sure you and Stephen have a good reason to say it's a pun one way around and not the other, but I really don't see it... perhaps the pun is such a quintessentially English kind of humour that you all have nuances and subtleties to them that escape non-native speakers.

A man walks into a bar
30/09/2012 12:55:15 PM
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Shouldn't the punchline be-
30/09/2012 07:04:15 PM
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How do you figure that?
30/09/2012 09:31:24 PM
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He's right
30/09/2012 10:30:01 PM
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Still don't get it.
30/09/2012 10:39:51 PM
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Is it?
01/10/2012 01:11:32 AM
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Three bearded men walk into a bar, wearing turbans...
30/09/2012 10:31:29 PM
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