For Yahoo! Messenger with Voice 8 users, it’s now possible to run multiple instances of Yahoo! Messengers and log into multiple Y!Msgr accounts with just a simple registry hack.
Multiple Yahoo! Messenger
If you prefer to edit and change the registry setting yourself manually, without using the above registry registration file, simply launch the registry editor (type regedit at Run command), and navigate to the following registry branch:
HKEY_CURRENT_USER\Software\yahoo\pager\Test
Then, on the right pane, right click on the registry editor and select “New”, then create a new DWORD registry value. Name the new registry key as Plural, and assign it the value of 1 (decimal). If some case, the value of 2 should also works. To disable or disallow multiple instance of Yahoo Messengers, simply change and set the “Plural” registry key’s value to 0, or delete the “Plural” key.