I just wrote something, realized that hey, Giles can't know that, and had to rewrite to make the same point more obliquely. Because I am writing something that has established itself as pretty solidly in his point of view. I haven't pulled out to be omniscient at all, or dipped into Ethan's viewpoint at all. I haven't so much as done a "Giles failed to see the giant spider crawling up the wall" sort of thing. If Giles fails to see it, I cannot mention it. This late in the story is not the time to start breaking that implicit contract with my readers. If I had done something like that early on, while I was setting up the rules for the contract, I could do it now.
I think it's possible that the rules are in place by the end of the first page. You almost have to do everything you're going to do in the first paragraph or two.
I am not a fan of head-hopping, that is, point of view changing from paragraph to paragraph. Point of view shifts can only happen on section boundaries. And almost certainly shouldn't happen in short stories at all. I'm not even sure a novella is long enough to sustain two different points of view. Probably it depends on the novella.
I recently re-read "The Dead" to pay attention to what Joyce was doing with point of view. He doesn't follow my rules at all. It's more of a cinematic thing, where the camera moves smoothly through the house, handed from person to person, and eventually settles on Gabriel's shoulder. (Though of course Joyce wouldn't have had this metaphor for it.) But he doesn't volley it around, either: no ping-ponging.
Reader expectation and fictional convention also factor in. As a modern reader, I expect stricter control over viewpoint characters. I think of headhopping as a sign of ... well, a new writer.
ETA: I think I'm being way unclear here. I do not object to shifting the point of view or having multiple viewpoint characters or anything like that! I think the important thing, to me, is that implicit contract with the reader I mention, about how you're going to do it. And yes, I confess, ping-ponging POV every paragraph or so does bug me. I will have to get more long-winded in a comment, or something. Or just go away and shut up. Sigh.