Thirteenth Sister and Tualon because they were the only ones with a remotely interesting relationship that didn’t consist of two cartoonishly evil henchmen bickering with each other in a cliche way. Their relationship was actually well-developed and although they weren’t good people by a long shot, their story made me care about them and hope that they’ll both be okay in the end.

Also I don’t remember the Second and Ninth Sister ever working together, or even interacting.

Remember when their first encountered Cal Kestis on Bracca?

Oops you’re right, I forgot about that.

They arrive on Bracca together at the beginning of Fallen Order, but yeah, once they find Cal they go on the hunt separately.

You brought up pretty much the only substantial example, and I can't comment on it since I haven't read the Inquisitor novel, but I do appreciate how most of these stories' portrayals of Inquisitors working together are based on the Fifth Brother and Seventh Sister in Rebels. Taking jabs at each other whenever they can, of course, but they do still know when to lock in for business.

I think the Obi-Wan series is the only story that overstepped the bounds, in its scenes with the Fifth Brother and the Third Sister arguing on the job. Nothing egregious, but it did dampen the intimidation factor the Inquisitors have over ordinary citizens and brought their conflicts with one another to a lesser depth.

The inquisitors are a bad joke.