Here is a simple Windows registry tweak with which you can login to Yahoo! messenger with multiple ids at the same time.
1) Go to start->Run.. type regeit then press OK
2) From the left side pane of registry editor window go to
HKEY_CURRENT_USER -> software -> yahoo -> pager -> Test (click on it)
3) Now right-click on right side pane & choose new DWORD value
4) Double click it and assign a value of 1
5) Now close registry and restart Yahoo! messenger.
For opening another id, open or double click on Yahoo! messenger as usual which opens another messenger window. Likewise you can login to multiple accounts at the same time.
