You must have yahoo messenger 8 or later installed in your computer to perform these tricks
Follow these Steps given below:
Go to start>>Run
Type regedit then press enter.
Navigate to HKEY_CURRENT_USER-->Software-->Yahoo-->Pager-->Test
On the right side of the page, right click and create a new DWORD value
Name it Plural.
Double click and assign it a decimal value of 1 (Change the default hexadecimal to decimal)
Close Registry editor Restart yahoo messenger then use muliple login.