Well, Harold McGee says, in the 2004 update of On Food and Cooking:
Because a standard kitchen extraction of eight hours releases only about 20% of the gelatin in beef bones, the bones may be extracted for a second time, for a total of up to 24 hours. The resulting liquid can then be used to start the next fresh extraction of meat and bones.
That sounds like something that would make sense in a restaurant kitchen that was making broth all the time, but maybe not at home. Worth a try, I suppose, if you have the refrigerator / freezer space.