For enabling multiple logins in your Yahoo! Messenger, Just Follow these steps :
*Click Start Menu And Click On “RUN”
*Type “regedit” and click OK
*You will see the REGISTRY EDITOR
*Navigate to “ HKEY_CURRENT_USER/Software/Yahoo/Pager/Test “
*Here in right side panel, right click to create a new Dword value with name Plural and give it value 1.
*Close the registry editor and enjoy multiple yahoo logins.