Homer, I have done some models by Boyer, the inventor of the DDRR idea. His design was for naval use over seawater aboard ships.
I scaled the model down to 27 MHZ with the vertical section connected to Earth. I got similar, very low gain results -17 dbi...just like Boyer reported at much lower frequencies. It was reported that the Navy used several 5,000 watt transmitter in series to get successful Tx results. The primary claim for this design was superior Rx results at very long distances. It was said, this antenna could hear crickets 1000's of miles away, when the rig was over sea water. Thus they had to generate big watts to be heard on Tx.
I also tried to duplicate Uk's Antenna Technical Author, Peter Dodd's, G3LDO DDRR mobile setup at 14 MHz, but had to make mine over a traditional horizontal 16 radial ground plane instead of using a vehicle type object for ground. My model produced horizontal RF gain at 2.09 dbi @ 5* degrees, which is very good for a mobile 5" inches above the Earth. However, the vertical side showed -20 dbi gain like the DDRR original design suggested.
Besides this mobile would be difficult to tune with all the moving parts...makes the idea impracticable IMO.
Homer, if this idea had any real merit, I think somebody might have tried making one. IMO this idea for mobile CB applications is like a boyhood pipe dream, with all the fire, sparks, and smoke that gets these guys all excited.