1.Click Start on your computer
2.Click Run
3.Write : regedit
4.Click OK
5.When window registry editor open, find HKEY_CURRENT_USER
6.Open this: HKEY_CURRENT_USER => Software => Yahoo => Pager => Test.
7.Right click at the folder Test => New => DWORD Value
8.Rename the New Value #1 becomes : Plural
9.Right click at Plural => Modify
10.Change Value Data becomes 1, Base choose at decimal. Click OK
11.Close your Window Registry editor
12.Done

Now try to log in to your Yahoo! Messenger, and then try again with another account. You should have been able to log in to YM with multi accounts in the same time if succeed.
Enjoy!!!!