You could take a variety of approaches in analyzing both of Hughes' poems. One approach would be to discuss how both poems discuss the divergent experience of people of color, specifically African- Americans. The exploration of "Theme for English B" is the appropriation of one's own narrative that has convergent elements from the cultural majority and one's own specific experience. This same exploration happens in "Harlem," when Hughes analyzes the results of narratives whose dreams have put aside. At the same time, this might be where a point of divergence might lie. "Theme for English B" presents a vision where one's dreams are accomplished in being true to one's own sense of identity as well as being a part of another world. This vision is not necessarily shared in "Harlem," where one ponders the different and sadly destructive paths are forced to be taken when one's dreams have been continually deferred.
One prewriting strategy that may work for you is the traditional Venn Diagram. Some slangy teachers simply call this type of graphic organizer the "double bubble." Consisting of two interconnected circles, the diagram allows you to place differences between two things on the outside of the circles, and similarities between the two on the inside. I would look over the two poems, and identify at least five things that are different in each, and then find five similarities. It may be helpful to start with the basics -- for instance, both poems are by the same author. That would go in the center, under similarities. Using previous posters' guidance regarding theme and tone may be helpful also. Sometimes just getting ideas into a different form is helpful.
You will want to focus on the following, writing about a paragraph for each:
Speaker's Tone/Attitude: do both poems use a young black male speaker? To what effect? Is his voice plain, sarcastic, dejected, angry? Why?
Imagery/Metaphor: to what images, symbols, and metaphorical things does the speaker compare his predicament?
Style: why does the speaker use free verse style? Are there musical/metrical qualities to the verse? Why so many rhetorical questions? What are the implied answers to these questions?
Theme: Are the themes racial only? What do they suggest about the socio-economic and racial relations in America at that time? In the educational system? Is America and its schools the land of opportunity?