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 pane , right-click and choose new Dword value .

4.Rename it as Plural.

5.Double click and assign a decimal value of 1.
That’s it done!!..

Now close registry and restart yahoo messenger .For signing in with new id open another messenger .

Credits from: techtrix