In accord with the first post, there is a clear difference in tone between the two poems. For one thing, the introduction of the fly suggests Beelzebub, the "lord of the flies," or the devil. This symbol of evil stumbling "Between the light and me" suggests that there is a point in which the soul of the speaker "could not see to see" where she is headed in eternity, whereas in "Because I could not stop for Death," the driver of the carriage that takes the speaker to her grave is "kindly" and "knows no haste"; the death holds no terrors as in the other poem; in fact, it is almost seductive.
In addition to the tone of the poems, you may wish to compare/contrast the sequence of events and poetic devices in the poems, the sort of expectations set up by phrases such as "last onset," "the king" and "be witnessed" in "I heard a fly buzz" with the phrases in "Because I could not stop for Death" such as those in the first stanza, "We passed the setting sun/Or rather, he passed us," and others that indicate a more leisurely trip toward eternity, whereas in "I heard a fly buzz," the predatory fly waits to claim a corpse.
Yet, characteristically of Dickinson, there is no enlightenment at the end of either poem. The speaker is driven, albeit leisurely, inexorably to her grave; the king witnesses the death, but he cannot control anything but the allocations of the speaker's material possessions.
In terms of working on this particular prompt, I think you are going to have to do much of the legwork yourself. Guidance and input can be given, but the bulk of the work is going to have be self initiated. I think a good starting point would be to discuss the mood of each poem. What general sentiments or feelings are brought out by reading each? This might involve you reading the poems to yourself, aloud or silently, a couple of times in order to grasp where Dickinson's mind is and how she attempts to connect to the reader. Another mode of discussion could be how death is characterized in each. What does death "feel" or "look" like in each? Then, ask yourself how different this might be from the traditional conception of death. What Dickinson is saying about death in both might be another topic to pursue. Finally, I would examine the theme, or overall message, in each. These might be good starting points to show points of convergence and divergence in both poems.
In order to get a good answer, you should ask a more focussed question. Are there specific things we can help you with? I'll give you a couple thoughts about the two poems that might help you get started, though.
The main difference I see between the two poems is in their respective views of death. In "Because I Could Not...," death is kind of interesting. It's a journey (literally, in the poem) where you get to see things and remain conscious for eternity.
By contrast, the view of death in "I Heard ...." is much different. In this poem, death is a much more negative thing. When the speaker dies, that is the end. All that happens is that she can no longer see. There is no vision of what happens next.
As I said, though, the more specifically you can ask a question, the more useful our answers will be.
Emily Dickenson's two poems are both about death. Death in "Because I Could Not Stop for Death" is kind and appealing. Death comes to get the person to take him/her on a journey in a carriage. The person is not dressed for the occasion as she experiences a chill. She is thinking back on her life and experiences as she is on her journey towards a new beginning. The poem presents death as a cycle and not an end. It is a comforting poem with a positive outlook of death.
In the second poem "I Heard a Fly Buzz when I Died" is about the preparation before the person's death and the final moment. She has prepared her will “Signed away / What portion of me be / Assignable—”)and now she waits for "The King" to come. She hears the fly which will soon secure his place with her in her grave as she decays. In the end when the window closes she has died and there is nothing more. The poem is a grim account of dying and does not speak of any tenderness.