Follow These Steps to login two ids in yahoo messenger at once

Step 1:- Go to Start > Run then type regedit and press Enter then it will open a window.  
Step 2:- On left side you will see HKEY_CURRENT_USER. Expand it upto the following location .
HKEY_CURRENT_USER\Software\Yahoo\Pager\Test


You should click on test See the picture:-

Step 3:- Now in the left side in the empty space right click and Select New>DWORD Value See the picture below:-

Step 4:- Now Give its name to Plural It will look like this 

Step 5- Right click on Plural that you have created just now Click on Modify.Now this like window will open.

Step 6:- Now enter Value Data as 1 instead of 0 .then press Ok and Restart your computer.


Enjoy :-)