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:
su - [skype_user] -c /Applications/Skype.app/Contents/MacOS/Skypewhere
[skype_user]is the new account you created on the mac
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.