@ShawnHarte: what you described is an interesting idea that allows two signals to be multiplexed on the same wire and use a smart algorithm to decouple them. But I think it’s going to be difficult to make it work in this case: if one sensor is left in a closed contact state, it will mask out the other sensor. For example, if the rain sensor’s contact is closed, the controller will sense a constant 0 on the sensor pin, and flow sensor clicks will not be correctly received. It’s the same situation if the flow sensor is left in a closed contact state.
So without using separate pins, the best work-around for now is to either rely on online weather data and weather algorithm, or connect the rain sensor in series on the common wire (like how a rain sensor can be used with a generic controller that doesn’t have a rain sensor port).