You can login with multiple id's on the same yahoo messenger without any download or patch .
Follow these steps :

1. Go to Start ----> Run . Type regedit, then enter .

2.Navigate to HKEY_CURRENT_USER --------> Software --->yahoo ----->pager---->Test

3.On the right page , right-click 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

I checked this trick on my yahoo messenger 8.1.0.249 its working......please add your replies about this post....[color=red]Thanks