Managing shared product backlog items across multiple platforms

I am using TFS 2012 to develop a application that will be available as a website and a mobile application, i.e. Windows Phone, Android, etc. While I've been building up a list of features for this application I've noticed that a lot of them will be available across all platforms and I'm not to sure how to manage them within a product backlog.

For example, there will be an option to sign in with a Facebook account and user will be able to do this on website and mobile applications.

So my though was I would create a product backlog item "Sign in with Facebook account" and assign it to an area called "Website". I would then create another backlog item, with the same title, but this time assign it to an area called "Windows Phone". Therefore my backlog would have two items, both with the same title, but different areas.

The idea is I could assign the "Sign in..." backlog item for the website to one sprint and then assign the "Sign in..." backlog item for Windows Phone to another sprint.

Seeing as I'm new using Agile/Scrum would this be considered a viable way of managing a product backlog?

Solution

I think each one should be a different story. Why? You'll probably have a different acceptance criteria on each one of those stories, and you'll (hopefully) use a different layout for web and mobile.

Some of the benefits of breaking stories into small chunks are:

Your testers will have stories to test from the 2nd day of the sprint, rather than get a batch on the last day of the sprint.
It increases the flow (I know that this is more kanban, but I think it's a good symptom)
It becomes more difficult to have a story that is in progress at the end of the sprint.

I've found this mind-map really useful to guide me to break stories into the smallest possible deliverable unit.