wonder why this feature is not by default included in Google Talk. One can login simultaneously to more than one account. Unlike yahoo multiple login no need of registry editing here.
Follow these simple steps

Create a shortcut of your Google talk.
Right click the shortcut ----> choose Properties.
In the "Target" you will find this
"C:\Program Files\Google\Google Talk\googletalk.exe"

Assuming you have installed in C:
Now at the end of target add this "/nomutex" without the quotes.
It should be like this
"C:\Program Files\Google\Google Talk\googletalk.exe" /nomutex

There is a space after googletalk.exe"
Thats it .Click OK and you can open more than one Google talk.
Amazing, isn't it?