Do you know Yahoo! messenger has built-in support for multiple login but its hidden and is not enabled?
By default you can only use a single Yahoo! ID to log into Yahoo! messenger and if you try to run another instance of messenger, it’ll not run.
But here I’ll tell you a very simple and small trick to enable multi-login feature in Yahoo! messenger and you’ll be able to run multiple instances of Yahoo! messenger and log into separate Yahoo! accounts without any problem and without using any 3rd party utility.
Following is a screenshot of running 2 instances of Yahoo! messenger with 2 different yahoo! accounts:

So here is a step-by-step tutorial to enable Multi-login feature in Yahoo! messenger:
1. Open regedit and goto:
HKEY_CURRENT_USER\Software\yahoo\pager\Test
2. In right-side pane, create a new DWORD value Plural and set its value to 1
3. That’s it. Close registry editor and now you can start multiple instances of Yahoo! messenger and log into multiple Yahoo! account in a single system.
PS: If you don’t want to do the task manually and want a ready-made registry script, then download following ZIP file, extract it and then run the extracted REG file. It’ll ask for confirmation, accept it and you have done:
Yahoo Messenger Multi Login