So, today i gave it a try. I have added second IP to my Openfire server's box. I have installed Stun server plugin on Openfire and configured it with primary IP and secondary one. I have disabled firewall on the server, rebooted Openfire. I have also set my Openfire server's xmpp domain name as Stun server in Spark > File > Preferences > Media Settings.
When trying to call, it shows "Establishing call" for a few seconds and then "Voice chat ended. No Media Received". There is also a Symantec Endpoint Protection on both machines, which can be blocking it, but i'm not so sure about it. Usually it doesn't interfere and file transfers work fine.
Testing with Spark 2.7.5 build 780 on both ends. Openfire 4.0.1. Stun server plugin 1.2.0.