buying app iPhone

Transfer the app to iTunes (if you havent already) or just download it to your other phone. (How many iPhones does one need? One for each hand?)
MacPro Dual Core 2.66 Ghz, MacBook Pro 15", iPhone 4 Mac OS X (10.6.6) AEXs, Red 2G & 5G Nano, Red Shuffle, 160 GB Classic, Apple TV

All apps on your iPhone and iPad should be in your iTunes library on your computer.
You didnt have to pay for the app twice to have it on your iPhone and iPad, so you get the app on your other iPhone in the same way it was installed on the other iPhone and iPad. Select the app under the Apps tab for the iPhone sync preferences with iTunes followed by a sync, or download/install the app direct on the iPhone using the same iTunes account that was used to purchase the app.
 MacBook Pro 15" Intel Core 2 Duo/2.53GHz/4GB RAM/500GB HD Mac OS X (10.6.6)  iPod w/Video 30GB,  iPhone 4 32GB