Tying 2 Fire Panels Together

I’ve always wondered this, but does anyone know how you would tie 2 or more panels together? So that basically one trips the other. My former high school had 3 panels; an Edwards 6500, an Edwards 6616, and a Mircom 1025T (last two in portables). I know that when they did fire drills, one custodian had to go to the basement to go to the 6500, and another custodian had to go to the portables. So I was just curious to how would they have set this system up so that the panels trip each other, if they do for that matter?

The basic way is to wire the alarm relay to a zone input on the next one.
NewAgeServer made a good video about it:

I would recommend wiring isolation switches to the zones on each panel used for the interconnection. Wire them so when the switch is turned off the panel goes into trouble. When you need to reset both panels you can turn the isolation switches off (both panels go into trouble) and reset them, then turn the switches back on.