There's two interesting points that I think Kattigan and Bolaire could bond over in the coming Seekers Arc 2
A fruitless search where very dangerous fruit is unexpectedly dropped in their lap.
Kattigan sent 6 years trying to figure out who the sorcerer that killed Marienna and Esmé were before his hope petered out into inaction the past year.
He went to magic institutions in hopes of getting an ID on this guy. And given it was Primus Tachonis and the Tachonis are helping finance at least the Penteveral and the Totality, I get the impression some people may have heard/recognized the description and thought "Nah. It couldn't be" and then said nothing. After all, why would Primus Tachonis personally find and kill and disappear the corpse a crofter's wife and daughter? And even if he did, do I really want to be involved?
So Kattigan searched and searched for answers, turning up nothing despite his great tracking skills. And then the answer drops right into his lap as he gets dragged before the Sundered Houses: the Lord of House Tachonis, one of the most powerful sorcerers in the world. The chances Kattigan can successfully avenge his family are slim to none at this time.
Bolaire has spent practically his whole life keeping an eye out for his siblings.
His 60 years as a weapon/battle slave are unsurprisingly unfruitful given he's only awake in short bursts to participate in battles. Yet once he has his freedom, Bolaire goes into the magical artifacts trade, not only securing himself a high position that makes it very socially acceptable to ask after other people's collections, but also getting into the underbelly/black market works too. He's placed himself to ask and hear all sorts of stuff about magic artifacts, magic masks, and yet he got nothing.
And Bolaire has been at the Archanade/the curator for 8 years. If Kattigan ran out of hope after hunting for 6 years, I don't think it's unreasonable for Bolaire to have at least backburnered, if not also lost hope in, his search for his siblings after even longer of finding nothing.
And then Termina, who Bolaire thought was dead, jumped out of the Coffin of Olbalad to attach to Lady Cormoray. And because he doesn't really know how to talk with his sister (and a woman who wants him dead has her ear), Termina stands against Bolaire and wants him dead. Trying to mend that relationship is courting death, especially with their current power gap.
People's disbelief in the hurt and trauma they've been put through.
Kattigan has had so many people doubt his story of Marienna and Esmé's murder with all evidence disappeared that he has to fight a doubtful voice in his own head about his own memories. Kattigan is fighting the insidious planted doubt that his wife didn't just take their daughter and leave him because he's just a drunk crofter and maybe she wasn't happy with him.
And again, given the Tachonis are doing some magical research institution financing, I think there's a decent chance some of the people he talked to at such institutions looking for that identification did recognize the description of Primus and chose not to identify him as such, probably out of raw disbelief or fear of interfering with Primus's business.
And while I'm sure some people did honestly try to help Kattigan in his search, it sure seems like most people didn't even believe the story/events that sent him searching.
Bolaire has been hurt and traumatized by his dealings with Thjazi Fang. Regardless of intentions or slanted points of view, Bolaire clearly came out of that situation hurt and traumatized. And even when he bares that hurt, does not have his claim of Thjazi saying "tell it what to do next" refuted by Thimble, he gets told to let it go, ignore it.
Beyond Thimble offering/asking to hear stories she never bothered to ask for before despite knowing shit was going down, basically no one takes Bolaire seriously when he speaks of his trauma. They just gloss over it or tell him to knock it off. They give it little relevance or weight.
So yeah, I think Kattigan and Bolaire have a chance to bond over what's happened with their searches and the ignoring by others of their pain, given the similarities.