Not sure if I have understood what you mean by a "double-hooped design" Do you have any photos?
For a roll bar to be functional (ie protect the occupants in the event of a inversion) it should be braced at an angle which supports the main hoop. The only designs I have seen with 2 hoops are not braced so they will be much weaker.
Librands do a good system where you mount the lower structure to the car permanently, but the upper structure can be unbolted, however be warned, a bolted structure is not as strong as am all-welded one, and the permanent structure wil take up luggage space.
