Horizontal scaling configuration

Are the following broadcast config keys required to be set on both of the machines running Traccar software in order for horizontal scaling to work? Anything else to be set?

'broadcast.interface'
'broadcast.address'
'broadcast.port'

https://www.traccar.org/configuration-file/

Yes, you have to set it on all servers. There are other parameters available.

How do I check if horizontal scaling is really working?

Connect app to one server and device to another. Check that you get location updates and events.

Both machines are using the same DB. Is this allowed by design using horizontal scaling?

Not only allowed, but it is required.