OK let me have a try ...
I have done exactly this, but inside Flash.
Firstly, you need to put each gallery in a different folder. This should contain the viewer.swf, imageData.xml, slides folder and thumbs folder.
By changing the xmlDataPath INSDIE flash (line 7, frame 16 of the viewer.fla) to something like
gXmlDataPath = "galleryA/imageData.xml" and then output the viewer.swf, you will crate a swf that looks for the correct xml file.
Move this viewer.swf into that folder and you're away.
Repeat and rinse as necessary [/b]
Alternatively, you can keep all images in the same folder and change imageData.xml name to galleryA.xml ... but there is a LOT of renaming to do to keep them all individual. Much, much easier the first way.
So once this is done, point your particular button at www.yoursite/galleryA/index.html
The above is IF you have access to the flash code. This is the easiest and most repeatable method I found.
Overwriting the path to the loaded XML file by setting the '_root.xmlDataPath' variable - as suggested by Felix is another version. I just like to keep my viewer.swf specific to the particualr folders for later modification.
If you DON'T have access to Flash or the SOURCE CODE it's going to be a little harder. As the viewer.swf dictates the path for the imagData.xml file location, just changing/creating the path within the xml file itself doesn't work either. The viewer.swf will look in the root directory [default], not the directory it was called from.
As the viewer.swf calls the xml data from within, the image path for folders & thumbs inside the xml files doesn't affect the outcome.
It is possible to create a work around, believe me I tried ... BUT (and that's a HUGE BUT) my suggestion ... buy the source code. It will be WAY quicker. Factor in the time it takes to reprogram your html to work, and it's complicated to do so, and you've already paid for the source code several times over.
Simple Viewer is inexpensive comapred to other similar products, and you can modify stuff to your heart's content then !!