Heya
There are a ton of things you can compare, be it the similarities or differences. Jane and Bertha have very similar characters, they're both attracted by fire and undergo virtually the same events. Tia wounding Antoinette is a direct echo of John Reed hurling the book at Jane, for example. You can talk about Rhys' choices in creating her prequel, how she mirrors many things, but twists others. While their upbringings are similar, Jane's true change came from the presence of Miss Temple and Helen Burns, and Antoinette has no such figure. In her school, God is talked about within the same sentence as the proper treatment of cuticles.
That part was in terms of plot (duh), but you can also compare the writing style and narrative gaps. Like... in the first page of each text, on begins with the definitive statement of "there was no possibility of taking a walk that day", whereas in WSS, Antoinette's language is based on pure hearsay "some say that when the ... close ranks". So just through the language, you can tell the differences within the characters, which you can comment on.
I hope that part helped
And for the structure of that within the essay, the main marks go to JE, so work in a 2:1 way. Compare WSS a little during the first 2/3, but focus on JE. For the last 1/3, focus on WSS and the comparison
Good luck!