I had one of those on an old 10 gallon Cornelius keg I was given. I never tried to get it apart myself, but when I traded it into my local LHBS the owner was able to get the two pieces without heat/penetrating oil, etc. he used a socket on top and put the bottom portion in a bench vise. It didn't look like I took too much effort.
I know it's not much insight on how to do it, but I know it can be done.