Yes, it is now possible to run multiple Yahoo! Messengers simultaneously right after installing the official version of Yahoo Messenger. I used this to fool my friends. But some of them are tech-savy, so I won only 50%! 
Now we are going to do this without any third-party software.
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Now we are going to do this trick with the help of Windows Registry Editor. We are going to add some code to do it. Our aim is to merge this settings into Windows Registry.
Using The Two Yahoo! Accounts in Messenger
How To Run multiple Yahoo! Messenger simultaneously?
You will need to apply a simple windows registry patch to enable this feature.
Follow these simple steps :
1. Open the notepad.
2. Copy the following code
REGEDIT4 [HKEY_CURRENT_USER\Software\yahoo\pager\Test] “Plural”=dword:00000001
3. Save the above code as multi.reg
4. Either double click on this file or right click on it and select merge option.
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“registry editing has been disabled by your administrator”. this is what i get when i tried it..
Then log into administrator account, if you are not in it and try enabling Registry Editor !
good trick
Cheers !