I've seen this before, or something worded almost just like it, is this off of firestick's web site or something?
A few comments on parts of what was said...
First of all, co-phased antennas create a directional pattern that favors communications in front and in back of the vehicle. This is ideal for truckers and RV's that use their radios to speak to those on the same road/highway that they are traveling on. If you are speaking with someone in front or behind your vehicle and either one of you take a turn and leave the more powerful RF lobe, a definite change in clarity will be noted.
Not arguing what is happening just trying to put it in perspective. If you can get the antennas 9' apart you will get 1.1db gain over a single antenna in two directions (the line from in between the antennas or in front or behind the truck). It is often said that a gain of less than 3db doesn't amount to much. This gain is 1/8 of an s-unit in optimal conditions, which most vehicles are not. Anything less than a 9' separation will be less, and this less adds up very quickly. 4.5' for example would be .3DB gain (yes that is point 3 DB) over single antenna in the same two directions, so as you see what little gain you get drops off very quick.
Also don't forget, for any gain you get in those directions there has to be an equivalent loss somewhere else, in this case that would be the other two directions.
The second advantage of co-phased antennas is there ability to perform where there is little reflective ground plane for the single antenna to radiate its energy from. For instance, on fiberglass vehicles or those with light aluminum sheeting over a non-metallic frame. In those situations a co-phase set-up allows the antennas to use each other's radiation field to direct the combined energy across the horizon. In order to be effective at least 2/3's of each antenna must have unobstructed line-of-sight to the other antenna.
Just an addition here. This is true, a cophased set of antennas don't have to have a ground plane to work, however, they would work better with one.
One additional reason has to do with power. Stock CB's leave the factory with the transmitter power limited to 4 watts but there is an underground market of linear amplifiers in existence. Inasmuch as power will eventually turn into heat, and the antenna will be required to dissipate that heat, two antennas will more than double the heat dissipation abilities of a single antenna.
This may have opened up a can of worms. For one, not all power radiated from a CB (or any radio transmitter for that matter) turns into heat. If you have a descent antenna that is set up properly very little of that power is turned to heat. Also, the use of a higher quality coax such as RG-213 or LMR-400 to a single antenna can easily surpass the power handling capacity of two lengths of RG-59 coax. In the case of what is available for power output the two lengths of RG-59 coax used for most co-phased connections would be more of a limitation than a help.
Spacing of the antennas in a dual antenna installation will have an affect on the radiation pattern of the antennas. Technically, spacing between the antennas should be nine feet (2.7m) to have the most balanced pattern. If that distance cannot be achieved, that alone should not discourage you from using co-phased antennas as you will still receive most of the benefits of a dual antenna set-up. However, spacing less than five feet (1.5m) may require some special tuning skills in order to achieve satisfactory SWR readings.
As I mentioned above, 4.5 feet separation will net you .3DB gain (again that is point 3 DB gain) over a single antenna. 1DB gain is around 25%, .3db gain is insignificant, you may as well run just a single antenna at that point, that is if you have a proper ground plane.
The benefits referred to really come to one thing in my opinion, the antennas use each other for part of their tuning. A well tuned antenna acts as both an inductor and a capacitor at the same time. Inductors and capacitors have the opposite effect on the radio signal being transmitted. To get the most of that signal they need to be in balance, or resonance. The inductance comes from the length of the antenna, capacitance is from the ground plane. When you use a cophased setup (or any other form of phasing for that matter) the antennas also add capacitance with each other. This capacitance can be beneficial as far as tuning is concerned if you don't have a ground plane.
Word of note, this does
not replace a ground plane. As each antenna is half of an antenna, each antenna still looks for its other half. The antennas do
not see the other antennas in question as their other half, instead, if there is no ground plane they typically use the coax used to feed them. The lack of an actual ground plane will make the cophased antennas used in this way less efficient. They will still work, but not as well as they could work.
The DB
You could try insulating the metal pole from the mirror and see if it would function as a dipole set up. 
