iTunes allows you to use one license on up to 5 iDevices. As long as you use the same iTunes account, you can install on both your iPhones. There are two ways of doing this.
1) Connect your second iPhone to the computer where you sync your iPhone. Click the connected device in the left hand column, and then on the right go to the Apps tab. Put a check next to Navigon and sync.
2) Download directly to the iPhone from the App Store. It will, at first, appear as though you are purchasing again, but as long as you are signed into the same iTunes account with which you first purchased Navigon, before installing you will get a popup window that says "you have already purchased this application. Would you like to download it again for free?" Just tap OK.