I saw this question in a forum, but could not find it again. Here below is the answer from Apple's "Terms and Conditions." Thus you have to pay twice for in app purchases for multiple device.

Palli, I would rather pay $15 or $20 , etc. for IOOTP instead of $4.99 just so I can have all the season on my Iphone and Ipad at the same time...Just thought and hope for future editions.
IN-APP PURCHASES
Certain App Store Products may include functionality that enables you to purchase additional services, or licenses to additional functionality or content for use within the App Store Product ("In App Purchases"). In App Purchases that are consumed during the use of the App Store Product (for example, virtual ammunition) cannot be transferred among devices; can be downloaded only once; and after being downloaded, cannot be replaced. Once a consumable In App Purchase is purchased and received by you, Apple shall be without liability to you in the event of any loss, destruction, or damage. All In App Purchases are deemed App Store Products, and In App Purchases made within Third-Party Products are deemed Third-Party Products, and treated as such, for purposes of these terms and conditions.
In-App Purchases must be authenticated separately from authentication to purchase and download App Store Products by entering your password when prompted, but once you have authenticated In-App Purchases, you will be able to make additional In-App Purchases for fifteen minutes without re-entering your password. You can turn off the ability to make In-App Purchases on your iOS Device by following the steps outlined at
iPhone, iPad, and iPod touch: Understanding Restrictions.