Boot a server on default port 2302.
Boot a headless client on this server. (Defaults to port 2316) - Headless client has to on the same system as the server instance.

Boot a second server on the same machine using recommended port 2312.

Players will be unable to connect to second server due to Battleye port already being allocated to the first server HC.

Using a +12 port allocation sequence is no longer a good solution due to the extension of Arma 3 ports.

Suggested action:

A - Update Arma 3 Dedicated Server to recommend further port separation between instances. E.g

SERVER 1
2302 UDP (gameport + VON)
2303 UDP (STEAM query port)
2304 UDP (STEAM master port)
2305 UDP (VON reserved port but not used atm.)
2306 UDP (BattlEye traffic port)
so open ports 2302-2306
and leave at least 100 ports between the next server set

SERVER 2
2412 UDP (gameport + VON)
2413 UDP (STEAM query port)
2414 UDP (STEAM master port)
2415 UDP (VON reserved port but not used atm.)
2416 UDP (BattlEye traffic port)
so open ports 2412-2416
and leave at least 100 ports between the next 3rd server set etc.

or

B - Change port allocation sequence to greater than 12.