I'm sorry everyone, but I have a soft spot for these two. I know, I rambled a bit.
There are things they don't talk about anymore. The scar on the back of Phil's wrist, the reason Nick never sleeps, the one fight they had that wasn't triggered by work.
The story behind the scar is a good one. Phil had been a junior agent at the time, training under Nick with three others. It had been their first mission out, supposed to be achievable, even for juniors. However, what SHIELD's data had stated was a small-time terrorist group had turned out to be a rogue cell of HYDRA, in the progress of developing acid-based projectile weaponry. They made it out alive, and called for backup, but on his way out, Phil had gotten in the way of an acid-blast, meant for Nick. His body-armor had protected his for the most part, but some of the acid had run down and burned his wrist. Nick had been angry, and also blamed himself for Phil's injury, since the blast had been meant for him.
Nick rarely sleeps because Phil rarely makes him. He knows when his lover is simply tired, and when he desperately needs sleep; and he respects that. There have only been a few times his judgment has been wrong.
They've fought plenty of time, but it's usually small-time things, work related. Their only real fight started because Phil was too obsessive-compulsive, and Nick was too stubborn. It lasted for three days, even overflowing into the workplace, before Agent Hill told them to pull it together.
There are things they don't talk about, but they're few and far between.