This trick is shared on allforces.com. If you have bought one app and want to use it in another iPhone and dont want to pay for it, follow these steps.

Here’s a step by step tutorial on how to do it.

Scenario
I have 2 Macs, 2 iPhones, 2 iTunes accounts.

I used one account to buy a game, and another account to buy another one. The goal is to have both games playable on both phones legally with no hacks.

Solution

Have each computer be authorized for both accounts. Re-download apps on 2nd computer with 1st computer iTunes account.

Step 1: Authorize Computers

I step into my wife’s mac, launch iTunes, log out of her iTunes Store account and log in with mine.

Step 2: Log Out and In
Still inside iTunes, I go to the app store and click the Buy App button on the application I had bought in my computer.

Step 3: re-download
iTunes knows I already paid for this and just let’s be re-download it without charging me again.

Step 4: Confirm
The app then begins to download.

Step 5: downloading
Once the download is complete, it now shows in the Applications section of iTunes.

Step 6: downloaded
I can now log out of my iTunes Store account and log her’s back in.

Step 7: Log out and in

Step 8: Sync & Play
I can plug her phone in now and sync.

Step 9: sync
The app should transfer without any fuss.
Once synced, it shows on her phone ready for playing.

Step 10: play
And now both our kids can take over our phones and drain those batteries out.