It's a little-known fact that iPhone applications can be shared amongst family members or a select few trusted friends. The process is similar to that of sharing purchased songs within iTunes, and goes something like this:
- Make sure the Application you wish to share is on your computer. If you downloaded it directly to your iPhone, you must first sync with your computer to copy it there.
- On the computer which has the Application in question, open iTunes, go to Applications, find the app you wish to share, right (or control-) click it, and choose "Show in Finder."
- Copy the file shown to the second computer (i.e. the computer that syncs with the iPhone on which you'd like to share this application).
- Drag the Application file into iTunes on the second computer, then double-click it within the Applications section of iTunes.
- Give iTunes the password of the requested iTunes Store Account -- this will be the account from the first computer you originally used (in Steps #1 and #2)
- Sync the iPhone with the second computer.
That should do it. You now will have the App in question on both iPhones, and it should work perfectly. Again, this is similar to the music purchased on the iTunes store and will only work on up to 5 other computers.