Hi!
So, I don't know how much help this will be; I studied Streetcar for my A-Levels (I have just finished as of the end of June) but I did not do The Colour Purple. But, hopefully, the pointers that I give you based on Streetcar will help you for your second piece. I will walk you through how I used to approach essays in literature - I usually flapped when structuring them, so I am sure you will do a lot better than me!) but here's a general walkthrough.
Take your two texts and kind of hang out with them for a bit. Think of what you have learnt about them, and what the background to each text is. The general background of a novel is important when considering how to structure your essay. I won't give you answers because that won't help you, but think about Streetcar; Blanche and Stella come from the same upper-class Southern society at Belle Reve, but as the play picks up, Stella is more modern, in the 'mixing pot' of New Orleans (her marriage to Stanley is also indicative of relative cultural diversity) and Blanche very much lags behind this modernism. I don't know about The Colour Purple so you might have to fill in some gaps, but I have given you a couple of threads, so perhaps you can make some connections.
Now that you know loosely what to talk about, bullet point your points. Two for Streetcar, two for purple. A short point is all that is needed when planning. Under this, (imagine you are writing this down on paper) pick out 1-2 quotes which EASILY link to what your point is. You can use more quotes as your point becomes more fluent, but select 1-2 that you know show your point for sure. This makes your life a lot easier.
With these quotes, pick out what makes you come to that conclusion from your quotes. Has the author used particularly interesting language, is the quote backed by useful stage directions or monologue, etc.
Next, how can you use this quote to evaluate your point? Think about context - link in to your historical knowledge of the text (which I have briefly provided for Streetcar) and this can include background knowledge of the author. What was Tennessee Williams's background - who was he influenced by? Ernest Hemmingway is a good person to look into to make links, if you have the time.
After this, simply conclude your point. These interim judgements will be useful throughout your academic journey, particularly when it comes to exams. In exams, if you run out of time to do a formal conclusion, if your interim judgements are good enough, you may be able to get away with it. This can be the difference between an A and a C.
My lit teacher told my class that we HAD to have elements of post-modern literature engrained in our minds, particularly with the unseen prose portion of the exam, but it links to the texts you're studying, too. Post-modern is usually measured to be post-war, or with the mid-century; fashion, social norms, and politics, were all beginning to change. You can touch upon this with Streetcar, as the juxtaposition between Blanche's plantation owning, materialistic view on life seems much weaker and regressive than Stella's modern silhouette, in spite of her being in a coercive and abusive marriage, which in itself could be argued as an archaic model for a marriage. Regarding Blanche, the part of the play (when Stanley rapes her, towards the end of the play) when she brings out a soiled white sequined dress, could be a good physical representation of this point.
Please feel free to post more to this thread if you need any more help - I have some notes on Streetcar somewhere and I am more than happy to publish them to The Student Room if you feel that it would help you.
Best of luck