You can register two or more wow accounts within a battle.net account, but one mailbox name just for only one battle account.
In other words, different wow accounts can be combined to a single Battle.net account. You can do this by going into your Battle.net account and adding more wow accounts to it. Certainly, the premise of it is that both accounts are registered under the same user name. However, it is impossible to merging 2 wow accounts so all the characters that are available from 1 account.
Blizzard has forced all players to combine accounts into their battle.net account, which means that your account name is equal to the email address with which the battle.net account was made. You must log into accounts separately to access different characters, but there is a flexible approach that you can start up wow account twice, and log into a different account with each session.
Before registering many accounts you need to think that whether you need one or more wow account though you can set up many characters in the same combat.net account. Another choice is that you can register under only one name and transfer your characters from this account to another.