Apple is smart enough to make sure you don't do this sort of a thing. At least, I thought so.
The easiest way to share purchased apps is to share the phone directly. However, if you are reluctant to do that and would rather play the game on your own iPhone, you'll need to do some really radical thinking.
Sharing Purchased Apps Is Possible
Interestingly, you can share apps between two devices but there's no easy way to do that. You'll be – effectively – setting up a new iPhone in the process (in case your iPhone doesn't allow addition of a new iCloud account as primary).
Our first recommendation is a moral way of doing shared-apps. This would be a good solution if you've got a family that plays the same games, uses the same apps etc.
Method #1: Family iCloud Account
To do this, you'll be setting up a new iCloud account.
Step 1: Set up a new iCloud account from Apple's website or through your iDevice
Step 2: Now, share this email id and password to all members who'll be sharing the apps
Step 3: Purchase apps through this particular iCloud account *only*
Step 4: When someone sets up their iPhone or iPad or iMac, ask them to set up this iCloud account as the primary account (first). This way, all app downloads will be synced to all devices.
There are several pitfalls to this method.
#1: First off, a major problem you'll see is that messages sent to this account will be synced and pushed to all the devices that are setup with this iCloud account as primary. You can turn this off by disabling the switches (OFF) in Settings → iCloud.
#2: The second issue would be that you'll have to add your original (personal) iCloud account but you can't sync or backup data (photo stream, documents etc.) to this account from your iPhone. You can only sync notes, reminders and emails.
Still, if you really want to share purchased apps – because you've got an app-hungry family – this would be a good way to go.
Method #2: Reset, Sign-in with Other iCloud Account
When you reset your device, you are asked to sign-in with your iCloud account.
Alert! You say, “oh boy, that will erase all my data”. It will but if you've got them all synced and backed up to your iCloud account, you can get them back anytime. And note: this method is for the crazy ones.
If you sign-up with the other iCloud account where those delicious apps reside, you get them on your iPhone! To do this:
Step 1: Reset your iPhone in Settings -> General -> Reset -> Reset All Settings
Step 2: Your iPhone will respring and you'll be setting it up as a new iPhone
Step 3: Sign-in with the particular iCloud account (that has the app) and let the iPhone roll! All data will be synced and you will mostly have the app you are looking for.
Step 4: If not, head over to the App Store -> Updates -> Purchased Apps -> Not on this Phone and install the app!
As stated before, this is for the crazy ones.
So there you go; that's how you can ‘share' purchased apps between different iDevices.