I have experience with the Twin Coil Antenna. It's a 7.5" ferrite rod tuned loop and as such performs the same as any other 7.5" ferrite rod tuned loop, for sensitivity. Its main virtue is it can be placed remotely to somewhere better out of the AC noise field, if used indoors (such as near a window), and tuned remotely. I've also disassembled one and thoroughly checked it out on the bench some years back.  I found that the "twin coil" offers no advantage over a standard tuned single-coil 7.5" ferrite rod antenna, patent claims or ads notwithstanding.

For reward purposes in the ULR group, there are stock radios operated with nothing else, then there is the "unlimited" class where external antennas of any size are permitted or the radio itself can be modified to add a larger external ferrite rod such as the various excellent mods Gary DeBock has extensively written about. Ferrite Sleeve Loops (FSLs) are also popular external tuned antennas; their diameter and coil width determines their sensitivity just like for non-ferrite tuned antennas. Generally it's easy to exceed the sensitivity of a standard tuned 7.5" ferrite rod antenna with any type of larger diameter/larger area tuned air or FSL or even longer/fatter ferrite rod antennas.

73,

Steve