This isn't really a Skype or Mac specific solution but a general *nix way of running two instances of one program. You will need another account on your machine so if you haven't got one create that now. I haven't played around with the permissions on the second user to see what works but I am assuming they would need admin rights (open to feedback on that).
From the terminal:$ sudo su - [your other username]$ /Applications/Skype.app/Contents/MacOS/SkypeThis should launch a second Skype app but remember you are logged in as them! This mean for example that if you download a file it will go in to that users Download folder NOT yours! Of course if you are doing this regulary enough you could wrap it up in a shell script to make life easier