when you get your driver side antenna swr tuned and working the way you want it to then buy another coily just like it and a mount just like it
then mount it and take the other antenna off while tuning the other antenna
use 9 foot of coax from each antenna
then when you come to the back of the radio put you a T connector inline
then call it good.
when cophasing antennas at US cb frequencies the 75 ohm (usually rg59/u) cables from the antenna to the point where they couple via a splitter either at the back of the radio or a multiple of halwaves (12ft,24ft,36ft,etc) of 50 ohm coax (usually rg58c/u) from the radio should either be 6ft or 18 foot,6ft being a 1/4 wave q section and 18ft being the equivalent 3/4 wave 3xq sections or more correctly a 1/4 section + a 1/2 wave repeating section.
it makes no difference if you bring the cables together at the radio or multiples of a halfwave from the radio,what the q section does is step the 50 ohm off the antenna up too 100 ohms as in this case it is a 2:1 ratio transformer so that when both 100 ohms are joined in parallel the radio sees the 50 ohms it needs to transfer power efficiently.
each individual antenna should be tuned with a dummy load replacing the other antenna till both are tuned the same,at which point you can replace the dummy load with the 2nd antenna,this is equally applicable in base setup's or high power mobile setups where rg11/u would generally replace the rg59/u and rg213/u would replace the rg58c/u.
using 50 ohm foam type dielectric cables like rg mini 8 the halfwave repeating section is around 14ft 3 inches and multiples off that and the 75 ohm q section on 75 ohm foam dielectric cables like rg6/u would be around 7ft 1.5 inches or odd multiples thereof.the same dimensions would apply to heavy duty foam dielectric cables too for base/high power mobile setups.
it ain't rocket science.but its best to use high quality military spec cables and good quality amphenol or better connectors for best results.if you can't achieve the minimum 1/4 wave spacing off 9 feet on mobiles then dual antennas are probably a waste of money,a lot off hassle and won't show any worthwhile improvement over a high position centrally mounted
single antenna of the same type.all they will do is distort the radiation pattern,but they might look cool to your buddies,lol.
