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