Simply close your messenger if it is On and then follow the following steps :
1. Go to Start --> Run . Type regedit, then enter.
2. Navigate to HKEY_CURRENT_ USER --> Software -->yahoo -->pager-->Test
3. On the right page , right-click and choose new Dword value .
4. Rename it as Plural.
5. Double click and assign a decimal value of 1.
Wow you finished it.
Simply start the messenger and login your account.
once you sign in.
again click on the Yahoo messenger (From wherever you have started it last time)
it gives you another chance to login though you are logged in your first account.
this way you can now log into multiple yahoo accounts.