I don't remember exactly. But they're not far from Varric's usual spot. I can't remember exactly when they show up either.
Yes, they're the nobles nearest to Varric in the main hall.
It doesn't seem like there is a 100% chance for their dialogue to trigger. I reloaded a few times to choose different options for a romance scene to see the rest of the dialogue and walked past them each time, and only one out of the two times did they say the line.
Most of the time they just say "Madam Inquisitor" or whatever as you walk past.
I remember hearing three different conversations from them, but I only recall the words of the latter two (and this is rough paraphrasing):
First conversation they were saying that they didn't approve my choice and added that it was 'not very scandalous'.
"They're giving gifts now!" is the first part of the second convo (I can't recall the female noble's response)
"Oh, it looks like there might be a wedding gown involved before too long!" "Actually, there'll be two!" (Fem Inq + Sera in this case) is the third convo