How to share an app between two devices

You can download the app between two devices using the same iTunes ID (in Settings>iTunes & App Store), as explained here: http://support.apple.com/kb/ht2519. If they aren't using the same iTiunes ID, you can download it to your iTunes library or transfer it from the devices that has it (by connecting it to your computer, opening iTunes and going to File>Devices>Transfer Purchases), then sync it to the other device using iTunes.

If you're asking if you can share the same app data (game progress, etc.) between two devices using iCoud, you can only do this if the app is iCloud enabled. Look at the apps feature description or ask the developer if it syncs app data between devices using iCloud.