iMessage: "This email address is already in used"

Are you allowed to use the same email/id in imessage for multple devices?

I've setup up fine on my iPhone but when trying to add my @me account to my iPad for imessage i get 'this email address is already in use'.

it's automatically setup my other apple id @gmail which i use for purchases.. that means i have 2 separate imessage contact addresses which is basically pointless.