I'm not sure if it was previously possible via workarounds, but the latest beta 2023.2
introduces sensor groups (see pre-release notes).
You can now create sensor groups via UI or YAML configuration, which calculate a value based on your defined group type ('last', 'max', 'mean', 'median', 'min', 'range', 'sum'). For your use case, 'min' should work e.g.:
- platform: group
type: min # one of ['last', 'max', 'mean', 'median', 'min', 'range', 'sum']
name: Battery Sensor group
entities:
- sensor.first_device_battery_level
- sensor.second_device_battery_level
Using the attributes and state provided by the sensor group, you should be able to create the desired notification:
# attributes
icon: mdi:calculator
friendly_name: Battery Sensor group
entity_id:
- sensor.first_device_battery_level
- sensor.second_device_battery_level
min_entity_id: sensor.first_device_battery_level
unit_of_measurement: '%'
device_class: battery
# state
64.0