@sbrunner - correct, the rain sensor won’t delay the execution of the zone, but it will keep the available water balance correct, so the Rachio would fire again on the next schedule opportunity. See this post -> ‘Freeze Protection’ at 50 Degrees (F) Implemented around using the sensor port to prevent watering at freezing temperature, but keep the available water value correct.
The flow sensor is to help provide Rachio with accurate usage versus square feet times precipitation rate times time. If both Rachios are watering at the same time the flow would be double counted. I think due to common wires one would need to either use two flow meters, one for each Rachio, or use an isolator to isolate the commons from each other.