Here locate the location :

HKEY_CURRENT_USER/Software/Yahoo/Pager/Test















Here in right side panel, right click to create a new Dword value with name Plural and give it value 1.

















Now close the registry editor and restart your computer after any changes to go into effect. For signing in with new YAHOO ID open another messenger open as many messengers you need. If you want to disable the multiple messenger option then again open registry editor and change Plural value 1 to 0.