I just wanted to point out for future researchers of this thread, in order to get my cc_config.xml file to be read on Mac OS X Mavericks, I had to su to root and create the file in:
/Library/Application Support/Boinc Data/projects/www.primegrid.com/
I had to set permissions with:
chmod 755 cc_config.xml
And I also had to change the owner of the file:
chown boinc_master cc_config.xml
And then when I reloaded the config file using the client gui, the second GPU was recognized and the fans started really spinning!
thanks for the effort by all in this thread!