Apple let you allow to enable automatic download of books, apps and music to iPhone and iPad which you have purchased on any other of your iDevice. Through guide below you can learn how to turn ON automatic apps download on your iDevice over iOS 4.3.3+.
Enable/Disable Automatic Downloads on iPhone and iPad
Tap on “Settings” icon from Springboard and then scroll down to “iTunes & App Stores” and tap on it, Sign to Apple Store by using your Apple ID, if you are not Signed in already
Tap ON/OFF switch for each option to enable or disable Automatic Downloads, and just close the Settings