If you must, the safest way is to run two instances in different directories.
It should be perfectly fine to do what he's describing as long as you completely close down bitcoin-qt between moving around the wallets. That's what I've been told by developers.
Whatever need you think you have for multiple wallets, you are probably wrong, and there is a better way.
monocolor, if you want to describe why you want to do this, someone may be able to suggest a better way to achieve what you're trying to do. However, I can imagine valid reasons for doing what you're doing.
the amount I sent to inactive one I can not see it anymore, even I swapped the wallet file names and reload the qt.
I think that shouldn't happen, but starting bitcoin-qt with --rescan (on Linux; do whatever is equivalent if you're using some other OS) will likely fix it.