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 .