Yes, you can assign a single sensor to multiple zones.
In the Web App
To use one sensor for multiple zones, they must be grouped into a “Zone Group”. All zones in the group need to have the same zone settings in order for the “Create Zone Group” option to appear. This restriction is in place because in order to control and scale watering appropriately and consistently, Spruce needs the zones that are sharing a single sensor, to be similar.
In the Scheduler SmartApp
The same sensor can be assigned to multiple sensors. It makes the most sense to do this when the soil type, landscape type, and wind and sun exposure are similar between zones.
Spruce will calculate watering times based on the moisture data from its assigned sensor before watering begins on any of the zones, so there is no need to worry about zones skipping watering due to multiple zones being assigned to the same sensor.