Things to think about with this:
Single or Dual Fire Alarm Systems
The idea behind connecting two fire alarm panels together is to make a single fire alarm system out of two separate fire alarm systems. If one fire alarm system senses fire, then both systems sound the alarm.
Normally, if there's a fire, it would be nice if everyone in the building got the "Get Out of the Building" alarm. If the two fire alarm systems aren't connected together, some people won't be told to "Get Out" until the fire has progressed far enough to set off the second system. By that time, the fire will be moving fast.
If it's the Authority Having Jurisdiction (AHJ) that is requiring the two systems to be connected together, read:
This shows some insights on who is the AHJ, and what authority the AHJ has.