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If you mean sign into different Skype accounts on the same computer, all you have to do is sign out of one, and then sign in with the other. I do it all the time.
 
You could try copying the Skype app and launching the original and the copy. From the preferences each one would have to be set up to use different audio hardware for input if you plan on talking on two different Skype calls at the same time.
 
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