Yes, you can use both power sources at the same time. If device settings are not changed, by default it is "auto" mode - it will automatically detect what type of the device is attached and provide necessary output. If the PD works with both low/high input, you can specify which output from PSE you want to use, by default the device will be powered with high voltage.
Device itself does not have any onboard voltage converter. If the device will be powered with 24V PSU and/or 48V PSU - it will also provide 24V and/or 48V on poe-out.
All information you can find in link below:
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