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 press 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 successfully!!

now open as many  yahoo messenger and use them with multiple login..