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