This is very simple trick for windows xp users which let you open multiple yahoo messenger in same computer. It may be two three as many you want. I had not tried it but, It might also works on window 7 and windows vista. So below is the trick :
1. Go to Start > Run
2. Type in regedit and then press Enter .
3. Navigate to HKEY_CURRENT_USER > Software > Yahoo > Pager > Test
4. On the right side press right-click and choose new Dword value.
5. Rename it as Plural.
6. Now Double click and assign a decimal value of 1.
7. Now you can open multilpe yahoo on same pc.