How do you merge two World of Warcraft accounts together?
If you mean merging 2 wow accounts so all the characters are available from 1 account, that is not possible.
It is, however, possible to merge up to 8 WoW accounts under a single Battle.net account. You can do this by going into your Battle.net account and adding your second WoW account to it. This can only be done if both accounts are registered under the same name.
You will still have to log into an account separately to access different characters, but you can start up WoW twice, and log in to a different account with each session. Addendum:The above answer isn't correct. You can certainly merge 2 accounts as long as your name is registered to both. The only exception to this I believe is if one of the accounts belongs to a minor over whom you have guardianship.