Basically, Yahoo! Messenger doesn’t offer multiple instances. Use this tiny registry hack to use multiple instances. This works for all versions up to 9.0. We’ll be using Registry Editor and a DWORD value to override it’s default single instance.

Open Registry Editor.

• Navigate to
  

  HKEY_CURRENT_USER\Software\Yahoo\pager\Test

• Create a DWORD (Right Click->New->DWORD Value) Value  named “Plural” (without quotes)
• Assign it a value ‘1‘ and click OK.
• Open Yahoo! Messenger just as you would normally. Every time you open Yahoo! Messenger, you’ll be presented with a new instance.

To Disable Multiple Instances, simply remove the DWORD value you just created (or) assign it a value ‘0‘.