It is possible to open multiple yahoo messenger. All you need is to edit the registry and add a new value. Follow to steps give below to use it.
Go to Start -> Run -> Type regedit, hit enter
Go to HKEY_CURRENT_USER -> Software -> Yahoo -> pager -> Test
On the right pane, right-click on an empty space and choose New -> Dword value
Rename it as Plural
Double click and assign a decimal value of 1
Now close registry and restart yahoo messenger.
For signing in with new id open another messenger .