Hello
The concepts usually follow up from class 11th to 12th, that is, concepts you learn in class 11th are also used in class XII for most topics, specifically for science background. So doing them together will help you to understand and link better between the two classes.
So, yes it is easier to do back questions of class 11 and 12th together taking a topic at a time.
However it might become boring and frustrating at times, so its better to break the syllabus and identify which portions can be done together and which topics need to be studied seprately.

Good luck