I went through the relay logic on paper and see it it’s exactly as you said. I think I’ve come up with a simple, inexpensive, 2 speed dual fan control system. Hayden makes a dual speed controller for about $30. By adding 1 external relay, I can mimic the circuit that you have. A $30 controller with a probe and adjustable trip point and another $8 relay and a couple fuses and I think I can make it work high/low with 2 fans for about $45. Hayden output 1 becomes relay 1, Hayden output 2 becomes relay 2, the new external relay #3 switches from series to parallel. I’ve done a rough drawing, not nearly as good as yours lol, but I’ll post it soon.... making my electronics degree pay off.... lol.