@Martin: I understand your request, but it’s important to make the UI/app consistent with the hardware capability. If the hardware only allows one sensor to be connected to the sensor terminal, the UI/app should match that and should not present rain sensor and flow sensor as two separate options.
If you need separate sensors, you will have to modify the firmware anyways. So the UI doesn’t really matter because in the firmware you can allow as many sensors as you need — the UI just provides an easy way for you to modify the options and parameters, but you can always changes these directly int he firmware, or through the HTTP GET command, without having to go through the UI at all.