I would not recommend allowing Neapolitan mastiffs ever to be in a situation where they could fight! They will never forget and you will never be able to have those dogs loose together again. Unless you have seen a full blown fight with this breed you can not imagine how hard it is to get them apart, not to mention the damage they can do to you and themselves. They don't stop! Any act of aggression or movement from the other will cause the fight to continue. We have seen grudges go to the extreme where the dogs will chew at chain link fencing to get to each other. Age doesn't matter. Litter mates aren't fool proof either. It only takes a split second and you have a life long grudge. Keeping two or more of the same sex Neapolitans together is not wise, I am not saying it can't be done or it hasn't but the risk is very high.