Not trying to disagree with you, as I'm sure your conversation may have had that tone, but whoever told you that from JL audio was not Carl Kennedy, Manville smith, or Lucho himself!
We just had a nice one day long insider tour of JL audio:
http://www.avsforum.com/avs-vb/showthread.php?t=819899
Specifically, Carl and Manville were very straight forward in saying that although two F113s may help in certain rooms as far as a smoother FR, in sheer headroom and extension, a single Gotham will beat two Fathoms.....they even hinted that it will be close to almost 3 fathoms.
We had a chance to hear it live, and it is a monster.....period. There are some pics of the facility and our tour there. Both Fathom and Gotham are really outstanding products, but the gotham is truly a pretty neat animal......LFE speaking of course