In normal condition, yahoo messenger can only open for one login. There is a simple way to make multiple yahoo messenger and you can login with multiple ID. All you need is to edit the Windows Registry and change the value.
1. Start ->Run type regedit
2. Choose HKEY_CURRENT_USER/Software/yahoo/pager/Test
3. Right Click then choose new -> DWORD value and rename it Plural. Double click and fill the value with 1
Close your Windows Registry. Open yahoo messenger , after one window then double click your yahoo messenger aplication again and login with different ID. Have nice try...