For some reason Skype on the Mac won't let you run multiple accounts or sessions.

To run multiple Skype sessions/accounts at the same time:

• Create a new non-administrative user (making it the same name as the Skype account might make remembering what it's for off in the future easier)
• Switch desktops or login as the new user and run Skype
• Set the Skype password and select the option to “Sign me in when Skype starts”
• Logoff and log back in as your normal user account on the mac
• Open Terminal
• Run su - [skype_user] -c /Applications/Skype.app/Contents/MacOS/Skype where [skype_user] is the new account you created on the mac
• Enter the password for the [skype_user] account

The only oddity I've noticed is that the dock status for the second Skype instance will constantly show a status of “Application Not Responding” despite the fact that all functionality is available from the top menus.