You are right that the app / UI currently assumes there is only one sensor, but that’s only relevant to the UI — you can still modify the firmware to support rain sensor and flow sensor at the same time. Currently the code for sensing rain and counting flow sensor are exclusive: if the sensor type is rain sensor, it will bypass the interrupt function for counting flow; conversely, if the sensor type is flow, it will bypass the code to check rain sensor status. If you use two separate pins, then they don’t need to be exclusive. So basically all you need to do is minor changes to the firmware to make sure both the rain sensor code and flow sensor (interrupt) code are enabled.