Citadels: Can two Circus Tents be built in the same city?
I would say technically yes, but that it wouldn't be as much of an advantage as it initially seems. Going from a purely literal interpretation of the rules you are right that the Circus Tent is an action and doesn't count as a building until the end of the game. That would lead me to say that during play having more than one is allowable. However, having two (or 3 as there are 3 in the deck) of your city elements leading to ending the game being worth no points and not counting toward any of your bonuses (except perhaps the 4 points for being the first to complete 8 buildings) seems to limit the usefulness of such a tactic. Especially since they would have been worth one point in your hand at the end of the game anyway.

With all that said, it is enough of a rules interpretation required issue that disallowing it would not be any sort of stretch either as it may very well have not been intended to be used that way regardless of how it was worded. Also, I think once this tactic is used in a game or two I believe it would be revealed rather quickly whether allowing this is unbalanced or not.