There are programs such as AutoHotKey (autohotkey.com) that can send key pressed to all your clients.

A nice example script follows:

Code:
WinActivate, World of Warcraft 1
Send, account1{TAB}password1{ENTER}

WinActivate, World of Warcraft 2
Send, account2{TAB}password2{ENTER}

WinActivate, World of Warcraft 3
Send, account3{TAB}password3{ENTER}

...
Where:
World of Warcraft *number* are the names of your wow windows (if you rename them on launch)
account *number* are the names of your accounts
password *number* are the passwords for the accounts