SOP - UK department offers two distinct masters - I am interested in both
I am on the admission committee for a fairly prestigious UK MSc and see this kind of thing all the time (indeed, we also get quite a lot of candidates who apply to both our MSc and PhD). More generally, it is very common for (a) intelligent people to have a lot of active interests at once, (b) intelligent people who face many open doors to struggle to choose which one the like most. The admissions guys (who are human beings, not machines!) understand this, and applying to both programmes will almost certainly not upset them. 

What will upset them is any sign of insincerity on your part, or any sense that you are trying to game the system by feigning an interest in one of the topics. My advice is therefore:

Treat the two applications as separate opportunities to write the strongest statement you can for each. In particular, think about how you can use the statement to give verifiable evidence that you have a real intellectual interest in the topic. Simply saying "I have a lifelong passion for X" won't convince anybody, so look for ways to send a more credible signal. For example, you could talk about how some readings influenced your thinking, or about some open research questions you think are interesting and how they might be solved.

Likewise, if you talk about what you will do with the degree in the future, try to be concrete and show that you have thought about how the programme fits your needs. What are the specific skills you hope to acquire and what specific modules on the degree will help you to develop those skills?

In your particular circumstances, I think it might make sense to include one or two sentences at the end of the statement acknowledging the fact that you are also applying for another programme. But, again, my advice would be to aim for sincerity rather than trying to play mind games. It should suffice to say that you fell passionate about, and ready to commit to either programme should you have the good fortune to be admitted.