Hello,
You don't need a second base. But I don't think the kit exists without the base, you'll have to buy everything separately.

Hello everyone Thank you for your support.
Enclosed is the answer from Eufy:

"We apologise that the kit form cannot be shared for sale. We apologise for any inconvenience this may cause. We appreciate your understanding."

So not available individually. You have to make unnecessary duplicate purchases.

Probably not, but the sensors can be purchased individually.
There is the Security Motion Sensor-T8910, the Door and Window Intrusion Sensor T8900, and the Water and Frost Detector. These sensors are compatible with the Homebase S380 and S280.

No, a second base station makes no sense, better to buy individual components.