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just download the app from the app store again, it is free if you're using you itune store account.
 
^ You shouldn't even have to do that. As long as you are synching both phones to the same computer, the App will be in that computer's iTunes Library. Both phones should then have access to those Apps. You can go in to the iPhones menu (when hooked to the computer and iTunes is open) and manually tell it which Apps to synch. Pretty easy really.
 
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