This is about on how to open multiple Yahoo Messenger through registry hack.
Go to Start >> Run >> Type regedit
Then Press OK
This is the registry editor window.
Go to HKEY_CURRENT_USER >> Software >> Yahoo >> Pager >> Test
Right Click on any part of the editor.
Click New >> DWORD Value
Edit the created DWORD Value
Change Value Name to Plural
Then the value data to 1.
Close the whole registry editor window and try to open as many YM as you like.