I'm assuming that custom audio's DVC speakers are a way to get stereo audio from a single speaker - a speaker with separate connections for a left and right driver.
Or maybe it's their "Dual Speaker" - "Custom Autosound's dual dash speakers provide great stereo sound from your classic car or truck's factory mono speaker location."
That's it exactly. It let's you put a 2 channel speaker in the stock location. Each voice coil is fed separately from left and right channels and the woofer is fed (I believe) from both channels.
Sound wise, it's a great alternative to cutting up door panels or whatever in a convertible where there isn't a good spot for 2 rear speakers. The channel separation argument goes away when you are driving a noisy convertible. An alternative is mounting 2 smaller speakers on a 6x9 plate. I did this with the front of my '70 vert. (4x10" stock speaker size)
That works OK, but bass is lacking. I've had to add filters to block the lower frequencies to these speakers otherwise they distort a bit on bass notes.
The 6x9 dual coil speaker in the rear doesn't have that issue, although I did add low frequency filters to that too as I have a powered sub woofer under the rear seat that takes care of bass.
Honestly, all of this is kind of subjective. You take an old car with rattles, minimal sound deadening, noisy exhaust etc. and then worry about the fidelity of the sound system. Unless you are really concerned about how it sounds sitting still, most of the improvements have minimal results once you hit a certain level. Put some old ears in the mix (like mine) that have been abused over the years from racing, shooting, loud music, and factory noise (again like mine on all counts) and even more diminishing returns on high end stereo equipment.
No, that is not a "co-axial" speaker. Different animal."Dual Voice Coil" = Co-Axial speaker.
Co-axial or tri-axial speakers do have different voice coils for different frequencies, but have no provision for separate left/right channel. Dual voice coil has one voice coil for each channel. The dual voice coil isn't meant to be used with one channel factory radios. But, used with the stereo conversions or aftermarket stereo radios, they work great for installation in stock locations where you don't want to cut or add more speakers.