As for the dual antenna spacing, my dad's a ham, and he says that you really need about 10 feet before duals even think about doing anything, and 12 feet before they do anything better than a single antenna. Even OTR trucks don't have the width, but it's close. So, other than that it looks cool, duals don't do anything for you. Sorry.
The physics of it is pretty simple, though. Each antenna emits a round signal, like dropping a rock into a still pond. When you put a second antenna near it, about 12 feet away, the 2 signals bounce off each other, and push the emitted signal into more of an oblong shape. This shape makes your yakking transmit farther forward and back, and less side to side. This is so you can talk to someone who is farther awar from you than the average CB range of about 15 miles.
As for a radio choice, I like my Cobra. It's connected to a 4' Firestik mounted on the top of my headache rack, and tipped alightly forward so at highway speeds it stands up straight. I won't do duals until I get the West Coast mirrors.
Hope y'all can understand this,
Scott