So here is a step-by-step tutorial to enable Multi-login feature in Yahoo! messenger:
1. Open regedit and goto:
HKEY_CURRENT_USER\Software\yahoo\pager\Test
2. In right-side pane, create a new DWORD value Plural and set its value to 1
3. That’s it. Close registry editor and now you can start multiple instances of Yahoo! messenger and log into multiple Yahoo! account in a single system.