He may calm down.. but it worked best for me to have 3. dispersed the aggression of the tank. I would add a separator to the tank and see if the big one calms down. if not i would keep them separated, or add another one between the 2s sizes. I first had 2 fires.. one killed the other, got 2 more and they all managed much better. every now and then they would scuffle. and i have heard of pairs working but imo its hit or miss since its almost impossible to accurately tell male from female. I have kept a few other spineys in 3s that are notoriouse conspecific aggressors as well with questionable success.
Yes, they will eventually kill another spiney if they do not get along.. much like cichlids. I give mine one night. if they don't settle down to a somewhat truce... i separate the next day.