Recently read this on the forums!
“For all intents and purposes, assume that the game doesn’t really understand the concept of a WoW license. The account-level achievement system will all be based on the Battle.net account and not each individual WoW license attached to it.”
I think it’s safe to assume that account-level achievement system will be applied the same as the account-level mounts and the account-level pets. Which means I’m very very happy that all of my characters, no matter which account they’re on, will get to pool everything together, achievements and all!