Multiple login in yahoo Without Using any SoftwareYou can login with multiple id's on the same yahoo messenger withoutany download or patch .Follow these steps :

Go to Start ----> Run . Type regedit, then enter .
Navigate to HKEY_CURRENT_USER --------> Software --->yahoo----->pager---->test
On the right page , right-click and choose new Dword value .
Rename it as Plural.
Double click and assign a decimal value of 1
Its done!!
Now close registry and restart yahoo messenger and try Multiple Login