I'm applying to PhD programs in Physics in the US. I have two fields of interest: Particle physics and complex systems. I'm currently doing research projects in both of them. However, I'm still not sure on which I would like to pursue my PhD. I'm not sure if I should address this situation in my SOP. I will mention both research projects but I'm not sure if mentioning two possible areas of research will hurt my chances of getting into the program.
Most applicants to US doctoral programs have only a bachelors, and one from the US, where the undergraduate program is generalized, not especially specialized. So, admissions committees have a lot of experience with applicants who are a bit (or a lot) tentative about their future research direction. You are probably more focused than most, actually, having narrowed it quite a bit.
But note that you apply to a department, not a university. If you mean complex systems in the physics context you are probably fine to apply to that department. But if you mean it in the mathematics sense, then it would be a different department requiring a different application, with a separate SoP. I'll assume you have resolved this.
So, assuming you mean a single department, I'd suggest mentioning both, but if you have a preference for one, then state it that way: primarily interested in X, but with additional interest in Y. But if they are different departments, write an SoP specific to that area of study.
The other issue is that after acceptance you need to find an advisor and you may learn that you will get more help and direction from someone in one of those fields rather than the other. So early commitment to a specific area may not be in your best interest.
If you have only a bachelors on entry, then the early years will be filled with advanced coursework in most places and you get a chance to meet faculty and make a judgement about an advisor. With a masters the path is a bit shorter and decisions need to come earlier, but leave yourself some "wiggle room".
My own experience (long, long ago) was that in math I was happy with either Real Analysis or Topology. That was enough. The place I wound up bas big enough that it made no difference. Plenty of advisors in both (and other) fields.