How can I make sure that I am writing an effective comparative essay between two texts?

A technique to ensure this is by using a different coloured highlighter for each text and going through the essay highlighting when you focus on each text. Look at the mark scheme to see whether it wants and even split (some may want 50/50 but others 60/40) then check whether your essay matches the divide required in the mark scheme or whether there is too much or not enough of one colour.