Saving photos in two projects in Aperture 3

A master can only ever be in one project at a time. A version can be in any number of albums/books/webpages/light tables.

I think you can duplicate a master and move the copy, but I don't have a copy of A3 in front of me.

 

I tell you to File > Export the master to disk, and then File > Import in the other project.

That's right, as long as you have the Managed master files stored in the Aperture Library.
Referenced master files may be used by another project, library, or application.

 

I believe you can make a copy of a "master" and then rename it allowing two copies of one photo.

For example, you have a file called AW113.TIFF (or .CR2/.NEF, etc). You drag that to your desktop, before importing into A3. You right click on the file and select "copy AW113.TIFF" and then paste it on the desktop. It will name it AW1132.TIFF You can then rename it something else. Then you have two copies of the same file and then can import BOTH into A3. I have done this several times. You can also create a new Library (I have 4 different A3 libraries for different aspects of my photography) and have the same master in several libraries.