Dear Alessio ,
Netro automatically creates dynamic watering schedules according to local weather, so if you turn on the smart zone feature for your zone, the smart watering is adjusted automatically.
If you want to make sure two zones are watered on the same day, we suggest you create user programs and set Smart watering: Level basic. (Go to Settings->Zones->Smart Zone)
In this level, no smart schedule will be generated. You will need to add programs manually (go to Settings->Programs to add programs) and Netro will skip programs when there is large amount of rain(>0.2inch/ 5.08 mm) or watering regulations.
Here is an article about Netro three watering modes which might be helpful. https://netrohome.com/forum/posts/111