You can take the exams in any order but you won't get the certification title without both. Oracle did have upgrade exams for Java 8 but retired them earlier this year. I'd recommend moving on to Java 11 at this point, with the new 1Z0-819. It's a single exam and will get you the latest certification title.
Our new Java 11 Complete Study Guide contains everything you need to know for the 1Z0-819. There's a book promo for it this week in this forum in fact!
Taking a cert requires a decent amount of preparation. I recommend writing a study plan first, such as doing a chapter a week (or 2 chapters a week if you're in a hurry). As long as you plan it out, it's certainly obtainable.