I’ve never used that plugin!
However, from reading the topic at Discourse Who's Online Plugin and the code at discourse-whos-online/assets/javascripts/discourse/components/whos-online.js at main · discourse/discourse-whos-online · GitHub, I’m guessing it’s the same deal, but configurable via the settings (in this case, whos_online_active_timeago
).
As for how they interact…
Do you use the plugin currently? If your plugin setting timer is set to 60 seconds, there might not be any way to tell which system is setting the active border on the avatars.