As a rule, we do not recommend using NGP antennas in a dual antenna installation. First of all, there is no such thing as a cophase NGP coaxial assembly. In order to have dual NGP antennas, you must use two single NGP antenna kits and bring the coaxial cables together behind the radio with a T-connector. Performance wise, it is a hit and miss proposition that could involve a lot of wasted time and money. Sometimes it works .. sometimes it doesn't.
The problem stems from the strong radiation pattern created by the design of our antennas. In a true co-phase setup, each antenna will use the radiated field from the opposing antenna as a reflective element. That condition creates the two primary elements of a co-phase system. First, it will compensate for insufficient ground plane and second, will create the peanut shaped wave pattern that is beneficial to those who need or want maximum field strength to the front and rear of their vehicle. Even though a dual NGP set-up is not a true co-phased system in terms of operational characteristic, the reaction between the two opposing antennas, under certain conditions, upsets the singular balance that was designed into the NGP antenna systems.
The spacing of the individual NGP antennas appears to be the key element in our decision to bring awareness to the potential problems that can crop up when using dual NGP antennas. If it were only a matter of recommending a specific minimum/maximum spacing of the antennas we could simply specify that in our literature. Unfortunately, transmitting antennas will sense and react to practically everything that exists within its near (and strongest)radiation field. Those are the things that we cannot control because the possibilities are unpredictable. That is, one vehicle might be fiberglass over a wooden frame and another may have a metal frame. Likewise, one may have a aluminum roof and the other a rubberized coating over fiberglass. There will also be cases where an air conditioning unit or roof top storage compartment may disturb the magnetic fields that are created during transmission. We just cannot know all of the possibilities.
Our technical staff has been challenged by this problem and has worked with several end users trying to grt their system in order to achieve satisfactory SWR. In almost every case, each situation required a different fix. One would require a specially wound antenna, another a change in the primary NGP coaxial length and the next, a change in the length of the balancing stub coax. Each became a custom design project that utilized excess time and money.
Even though many of the in use dual NGP systems are performing with the satisfactory SWR required, those that do not end up consuming all of the value gained from the successful installations.