i have three archiCADs and two plotmakers in my dock.
to make the copies, as dwight said, go to your archCAD 'directory' (/Applications/ArchiCAD 8.1 folder/) and hold down the 'ALT' key. now click the archiCAD icon and drag it to a little way down the folder list. see the green '+' button next to the cursor? that means you're making a copy.
OSX will then create a new archiCAD application in the same folder and append ' copy' to the end (you can rename this as you want). then launch it like any other application . . .
HTH
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