A zoned system will split the 5 tons if both thermostats call at the same time for cooling. If you only need to mainly combat heat produced by lights, then the room with lights off won't call for cooling. And the motorized damper on the duct for that zone won't open, so all of the airflow will be to one room.
But if the lights on room is calling for cooling, and the lights off room calls for cooling, it will open the damper until the lights off room gets cooled, then it will close again, and all air back to lights on room.
Treating each space separately is best, but costly.