If I request two characters, A and B, then I want them both to appear 'on-screen' for a significant chunk of the fic, and their stories to intersect in some way. In the context of a 1k Yuletide fic I would probably expect, say, a fic from A's POV where B shows up for at least a few hundred words of that 1k, either in the story's present or a narrative flashback rather than just A thinking about them in their absence. If they're not directly interacting then I would at least expect A to be observing and thinking about B's actions, and for those actions to have some sort of plot significance rather than being just a cameo you could snip out and still be left with a perfectly solid A solo fic.

Another possibility, depending on the canon situation and the type of prompts I leave, might be for A and B to be separated but to each still get one or more scenes from their POV or otherwise focused on them, which are then somehow connected and come together as part of the overarching plot. But that's more of a tall order with a 1k minimum, unless maybe it's something like epistolary with a back-and-forth of short letters between them or a Five Things type structure made up of mini scenes.