If they both start eating and gaining weight, I predict Brad will have no choice but to remove one from the display tank eventually since the dominant one will try to kill the other. If the remaining one starts to gain weight and grows in size and lives for many months or years but dies one day suddenly, then its death will occur at night.
Of course, it can die from starvation or disease and it can wither away in which case death is imminent. But if it was gaining weight and growing but dies unexpectedly (which is reported frequently), it will happen at night.