Follow Below steps :
1. Clcik Start Then Click Run . Type regedit, then Press Enter .
2. Navigate to HKEY_CURRENT_USER ???> Software ?>yahoo
??>pager?->Test
3. Right-click on Test and choose new Dword value .
4. Rename it as Plural.
5. Double click and assign a decimal value of 1.
Its done!!
Now close registry and restart yahoo messenger and try Multiple Login.
N.B. If you think it is Useful, Please give your comments. Thanks