Facetime with more than one apple device
Do you want to place calls between two iDevices using the same Apple ID?

By default, all devices connected to the same Apple ID utilize the primary email address associated with that Apple ID. This means they'll all ring if a FaceTime call is placed to that email address. It also means you can't place a class between two devices, just as you can't use one home phone to place a call to your house and answer it with another phone. But luckily, Apple has provided a rather easy workaround for placing FaceTime calls on the same Apple ID.

To place a FaceTime call between two of your Apple devices, you will simply need to add a new email address to one of the devices. This will allow FaceTime to differentiate between the two and place a call to a specific device.

You can add a new email address to FaceTime in the iPad settings. Simply select FaceTime from the left-side menu and tap your finger where it reads "Add Another Email..."

Once you add a new email address, you must verify it by going to that email's inbox. There should be an email waiting for you from Apple with a link that will verify the email address. 

Source - http://ipad.about.com/od/iPad_Guide/ss/How-To-Use-FaceTime-On-The-iPad_2.htm