I have multiple Yahoo accounts and I always wished to use Yahoo Messenger with all these different accounts logged in at the same time on same machine. A couple of moments back I was messing up with my computer registry to get it done and finally I came up with a solution
( better say two solutions ).

There are basically two methods to get it done:

1. Application patch: This method is for naive users who have no experience of working with registry settings. Just download this zipped package and run the application contained within it after extracting. You’ll get a window like this :

Click on the “Enable YMultiple” button and you are done. Enjoy running multiple Yahoo Messenger instances.

2. Manual Registry changes: This method is for experienced users who have knowledge to work with registry. Just follow the undergiven steps :

CAUTION: Don’t mess up with any other registry setting. Just complete these steps and abort.

• Open the Windows Registry file ( Go to Start > Run > Type regedit,hit enter).
• Go to HKEY_CURRENT_USER > Software > Yahoo > pager > Test.
• On the right pane > right-click and choose new Dword value.
• Rename it as Plural.
• Double click and assign a decimal value of 1.
• Now close registry and restart yahoo messenger.

After this restart your Yahoo Messenger and you are done with this.