How can I use "A Rose for Emily" or/& "The Story of an Hour" to show how a specific death scene helps illuminate the meaning of a work as a whole?

If you do not have to use both stories, I would concentrate on Chopin's "The Story of an Hour."  It is more subtle than "A Rose for Emily," and Faulkner often relies on death scenes to advance his theme; so "Rose" is really not much different from many of his other works.

In "The Story of an Hour," you have two deaths to work with--sounds bad to revel in that, but it helps you as a writer!  If you focus on the theme ofthe chains of marriage suppressing a woman, you can tie both deaths from the story to the theme. The opening line of "Story" is

"Knowing that Mrs. Mallard was afflicted with a heart trouble, great care was taken to break to her as gently as possible the news of her husband's death. "

Why does Mrs. Mallard have heart trouble in the first place?  Many critics believe that Chopin includes this fact not just to make the story's ending more believable but also to symbolize that this woman is being stifled or suffocated by her marriage.  In the third paragraph, Chopin discusses Mrs. Mallard's initial reaction to the news; this foreshadows that she is not behaving as most widows do when they receive such news.  All of these factors suggest the previously mentioned theme.  Finally, analyze Mrs. Mallard's own death at the story's end and the doctors' conclusion about the cause of her demise.

If you want to analyze "A Rose for Emily" in connection to "The Story of an Hour," you can certainly discuss the theme of tradition versus equality (or freedom) found in both stories.  Both stories use their death scenes to promote this theme.  In "Rose," Miss Emily's macabre bedroom and the discovery of Homer Barron's corpse illustrate the triumph of Miss Emily (once seen as a pitiful, helpless female) over the male.  She gets the last word in other words. Additionally, Mrs. Mallard feel free when she believes that her husband is dead.  While you cannot argue that she triumphs over her husband, you could suggest that in order for her to feel free and independent she must be free of her husband just as Miss Emily had to be free of Homer's possible rejection and the confines of her society.

Hope that this helps!