Sometimes on a single pc we need to open more than one g-talk window when its a demand of your friends,but no you can’t do that.First you have to sign out yours for giving chance to next,but now your problem is solved,you can open more that one gtalk and use it simultaneously…follow some simple steps:

1)Right click on the Google Talk shortcut from start menu.
2)click on Properties.
3)Go to Shortcut tab on Properties window.
4)On the Target textbox, add in the /nomutex at the end of the line so that it looks like below (or you can simply copy and paste the below syntax and replace the original).
“c:\program files\google\google talk\googletalk.exe” /nomutex
5)Click on OK and thats it.