How can I know how best to structure an essay, especially with a multi-text question?

Essay structure is arguably both the most important element of a good answer, and the one which worries students the most! This shouldn't be the case, as actually the best structures are simple and flexible. Once you have the confidence to quickly plan the way your argument will unfold and your points will interconnect, you can save yourself a lot of stress, and, even more crucially, time in the actual exam.  There is no one right way to structure your answer when juggling more than one text. The best thing to do is to let your ultimate argument decide. Sometimes a basic author-by-author approach is the sharpest and most effective way, especially if you want to highlight, for example, a contrast between them, or the way one informs/leads onto the other. On the other hand, sometimes a "thematic" approach - in which each paragraph discusses both authors in relation to a specific subject or technique - is more suited to a more complicated discussion. The best advice I can give is to look at it from your ideal end-point: which format will enable you to engage in a clearly-defined discussion which develops?