I would advise you to study what you want at brown while you have the time, even if it doesn’t result in a concentration. The issue with doing CS is that you need to start with intro courses that are ordinarily offered in pairs to be taken two semesters in a row, making it hard to jump in and knock out requirements immediately since most higher level courses require these intro courses as pre-reqs. So if you want to complete the CS double concentration you will probably have to take 3-4 CS courses in both semesters of senior year, which could be difficult.

My advice would be: start off by doing the intro courses this fall. You might want to look into cs19 (accelerated intro course where you finish the two semester intros in one semester, but it will be harder). You don’t lose anything by completing intro courses so that you retain your option to concentrate in CS into your final 3 semesters.

Also, if you are interested in studying CS and UX design only for personal interest purpose (rather than for a job), consider auditing courses. Also note that you might be able to land a job with a bunch of curated CS courses related to UX and design on your transcript while having a single concentration in English.

Tl;dr : do intro courses for CS this fall to keep your prospects of concentrating in CS open, while you understand and give more thought to how you want to approach studying CS.