There are different ways of doing this, but the first I've been pleased with is the use of Quicksilver.
With Quicksilver running, invoke it (Ctrl+Space, unless you changed it). Type enough of the word "Archicad" to have it pop up on the left. If you have multiple versions, select the proper one from the list below.
Tab. Type "Lau". You should see "Launch Another Copy". Return.
There ya go.
If you don't see the "Launch Another Copy" option, quit Quicksilver, relaunch it, and try again. I have seen this happen after launching another copy, quitting it, and trying to launch again. Seems like a bug.
It isn't needed, but there are times when it's very convenient. You can quickly copy and paste from another project. You can open a placed module without closing the host project. It comes in handy.
Run multiple copies at your own risk. Keep your head on straight. Don't run two all the time. Don't open the same file in two copies at once. Quit the second one when you're done with it. Pay attention to where the second copy is on the dock. All these warnings go double for people who are easily confused, you know who you are.
Another tip: When quitting two ACs, let one quit all the way (the black triangle disappears from the dock) before quitting the other. If you quit them both simultaneously, you're asking for corrupted preferences.