Assuming you're sitting at the center of the horn you're listening to the woofers off axis and reflected. The diagram below while it looks nice isn't indicative of how the sound waves are actually propagating from flanking woofers in my understanding. I like that this design is coaxial and seems like an interesting configuration to time align the drivers. At the same time doing so seems potentially very difficult to solve properly given off axis nature of the drivers, horn loading characteristics, and higher likelihood of phase cancellation given the drivers orientations.
I thinks it's crucial to take wavelengths and how they sum into consideration, for each of the sets of drivers located on the horn.
Take the SH-50 woofers, they probably low pass somewhere around 300Hz. Which equals about a 3-3/4ft wavelength, or about an 11 inch quarter WL.
Given the SH-50's woofer ports are all within 11 inches of each other, all within the 1/4 WL, there isn't really any phase cancellation between flanking drivers. Maybe a couple of dB less than fuller summation at 300Hz than at 150Hz, but that's as bad as it gets (which is dang good imo!)
The good ole phase wheel to the rescue
I've built several sets of these synergy/MEH type speakers, and can attest through many polar measurements, that if anything, they combine multiple drivers better than any other type design i've tried.