How do you know the two antennas are not properly phased and not positioned optimally, given the ergonomics of a typical transmitter? I am pretty sure Andy has told us the straight scoop and with my 45+ years of RF experience, I believe him. If you can show me proof of your statements, I'll consider believing you. In the meantime. I'll stick with Horizon's explanations of how their products work.
By the way, some of your other statement about how the receivers work together are also incorrect. I talked the Rxer processing out with Andy a long time ago. When using multiple Rxers, they all are listening simultaneously and each Rxer's data is processed. First one with the right checksum is executed. And, ... why would you need a third antenna for telemetry when you already have two that are perfectly set up for the given RF wavelength? All you need is a little time displacement multiplexing with priority and the antennas can work for both datastreams.
You might want to search around a bit. All this info is on this site. Just trying to help you a bit here.