This is so basic I'm probably telling you what you already know, but I see so many examples of POV mistakes that it seems worth reiterating this fundamental technique of narrative.
When you begin a scene you are telling your story—and I'll explain in a minute why this is relevant for both fiction and non-fiction—you are of necessity speaking from someone's vantage point. Strictly as a hypothetical, if you start by saying: “Shelly had long considered the house inadequate. Now, speaking to John about why it could not be sold, she must make a different argument.” You have established he scene as being in Shelly's POV. You cannot then tell us what John thinks. You have instead to tell us what he says and/or what Shelly believes he thinks.
When you end that scene and you are no longer reporting that particular conversation you are perhaps free to invoke a different POV. (Unless, of course, you have set up this particular story as being in what's called limited third person. In that case you are telling the story entirely in Shelly's POV, but using third person: “Shelley had long considered.” Rather than, “I had long considered.”)
A really skilled technician can break this rule and tell a particular scene from multiple points of view, but doing so can really mess up your pace. Something you don't want to do.
Or you can elect a different approach and divide your story precisely according to who is doing the telling. In the opening of Gillian Flynn's runaway bestseller GONE GIRL she starts with Nick's POV and says: “When I think of my wife I always think of her head. The shape to begin with.” Later she will switch to Amy's POV and tell the same story from a very different perspective. The point is to make the reader consider which of the two major POV characters is telling the truth. Flynn thus played with what proved to be a game-changer in commercial fiction, the unreliable narrator. (Though to be fair that's a conceit employed by the likes of Paul Scott or E.M. Forster.)
Returning to the question of applying this principle to non-fiction—whether article or book length—you still have the same considerations. In non-fiction you're more likely to have begun by immediately establishing who Shelly is and why we should care what she thinks about a particular house, whereas in a novel you might choose to let those reasons unfold as the drama moves forward. But in terms of POV it comes to the same thing.
Readers want to be—indeed almost always need to be—in the head of one person at a time.