Is it allowed to use third-person limited narration that focuses on the words, actions, and possibly the thoughts of two characters within a single chapter, part or scene of your story?

Of course, but is should be carefully done. I’ve switched perspectives within a conversation before, in order to clue the reader into things that one or the other character didn’t know, or just to show how each was reacting to the discussion.

For example, in the Measure of Man, which I’ve been writing, most of the story is from Johnathan’s perspective. But sometimes, when he’s discussing things with Strux or Kazya, I’ll slip into showing things from their perspective. Usually, in Strux’s case, showing how the human is starting to make sense. Or, in Kazya’s, as part of a bit of humor where, being from an all-female species, she simply assumed that Johnathan was likewise female because nobody had told her otherwise. However, it did also show an interesting cultural perspective.

The key, really, is simply to make sure that the reader understands firmly from whose perspective the current bit of the story is being written.