If you are really not sure between those two interests, then I think you should be honest about it. For example, don't apply to Princeton Astrophysics saying that you are "most interested in computational astrophysics" and then also apply to Princeton Physics saying that you are "most interested in theoretical high energy physics". There may be faculty who are affiliated with both departments and might see both SOPs!
I think that you have two options:
1. Decide which department is the better fit for you and strongly state your preference for that topic. Maybe you won't even apply to the other department. (Also, some schools will let you be enrolled in one department but work with advisors from another)
2. Let both departments know that you are interested in both topics and give very good reasons why. In the SOP for each department, explain why that particular department is a good fit for you without comparing them to the other department.
I think (1) is a good idea because (2) might make you appear unfocused and each department might decide you are a better fit for the other one. However, in many cases, it's really fine to have two research interests (and there is some overlap in your interests) so as long as you still demonstrate that you are knowledgeable about your interests and show that you would fit in, (2) is good too! Basically, all I would really recommend is if you are applying to both departments, don't tell each department that they are your favourite etc. Assume that they will see both SOPs even if it's unlikely.