How should I structure a comparison essay?

For an essay where you are comparing two texts I would recommend comparing both texts throughout the whole essay in order to show clear engagement with them together, as supposed to writing solely about one text, and then another. If you are expected to write and essay in about an hour I would say pick three points and identify how they can be applied to both texts. I would begin a paragraph by stating a point, like a topic sentence, and then going into details from one text in order to back up this point, using quotations, setting etc from that text to provide evidence. I would then link this directly to the other text by highlighting similarities or ways in which the two texts contrast in light of the certain point.
For example in an essay about masculinity in 'A Streetcar Named Desire' and 'A Duchess of Malfi' you might begin by stating: The men in both plays have a strong need to assert their masculinity in order to intimidate women. This is seen through Stanley in Streetcar in his treatment towards Blanche....(provide evidence). A similar relationship is shown between the Duchess and Ferdinand, however the power of Ferdinand's masculinity is less than that of Stanley's as the Duchess shows she is unafraid of her brothers dominance, unlike Blanche's intense fear of Stanley. (then go on to provide evidence to back up these points).