You'll need to use a cloud service, such as iCloud to store your shared data. There is no sharing mechanism with a Universal App to share data between devices. The Universal App just lets the same App work on both iPad and iPhone. This is a nice bonus to the customer, because if they have both devices, they only pay for the App once.
The App will need to go get the data from the users storage. If you don't want to use iCloud, you can develop your own, by using storage services from Amazon Web Services, or Microsoft Windows Azure, however, you are stuck with paying for the bandwidth and storage on those services. You're also getting yourself into more work as you'll have to come up with the server side support, and web services yourself. Plus, on the mobile device, you'll need to concern yourself with handling losing a network connection, caching the data on the device, and then sending it when connectivity is regained.