How do I get multiple versions of Unity running at once

I want to have two instances of Unity (free) running at once. That way I can look at reference/tutorial material in one and have my own project in another.
But when I try and do this Unity gives me a fatal error saying it can’t open the same project again. I never asked it to open the same project again, it just seems to think I do - so how do I get a second instance of Unity running concurrently?

Unless you mark “Always show project wizard” under preferences it will open the last project you opened (which is the project you have open in the other instance). I’m pretty sure you cannot open the same project with two instances of Unity. Even if it would let you it would probably crash because there are resources that would get corrupted.

You can press Alt when opening the Unity Editor. This will open the project wizard instead of opening the last project. Ray is correct in saying that you cannot have two instances of the same project open (unless you’ve copied it to a different folder… which makes them different instances).