You need to link your two devices together with a Google account. Done, hope this helped!
Okay, I'm not going to be that guy...
Here's how to really do it:
- Open your Settings app.
- Scroll down to find an option called something like Accounts and tap on it.
- Now find an option that says Add account.
- Now click the type of account, in this case, a Google account.
- Now sign in with the same info used to initially buy the app or game.
- When you're done, go into Google Play and swipe right to enter the options for the app, or click on the three-line-symbol in the top left.
- Click on your current account to switch it out, now click on the account you just added.
- Find the application you previously bought, and it will say Purchased.
- Download and complete!