I have Facetime on my MacBook and Facetime on my iPad. Of course I also have one Apple ID account. Sometimes the family uses the iPad while I'm away. I'd like to be able to call "myself", that is my iPad from my MacBook. How do I do this?
The short answer - you can either call a phone number (iPhone + FaceTime) or an AppleID (iPod/Mac/iPad + FaceTime) and you can't call another device with the same number.
To call back to your mac (or any other AppleID device) - you'll have to make a second account with a different Apple ID. Similarly - there's no easy way to get the phone number into another device since that gets authenticated over SMS in the background and not user/password for AppleID.
If you are calling from an iPhone - you can call your AppleID devices (they all ring and any one can pick up), but for AppleID based calls - you need to call a different AppleID unless Apple ever changes the current software.
Last clarification - AppleID is just a string but most people use an email address since that is more likely to be unique to one person.
In this case I dont think it is possible with the way Facetime is configured.
Facetime has some sort of reverse dialing mechanism associated with your apple id. That means you need to dial out to a different unique id.
If you create a second apple id for this purpose and associate it with the iPad or the macbook, you should be able to call them when you are away.