I do Chemistry and Biology.
Obviously it depends on the exam board, but for AS edexcel Chemistry it starts off quite straightforward - in fact, unit one really isn't much harder than GCSE single-award Chemistry. In unit two you study some areas in more detail, such as organics and kinetics/equilibrium, but the difficulty level isn't too high. Rather than a single coursework assignment, you complete a series of assessed practicals and then sit a short practical-based exam which tests pure recall. As for A2, I obviously haven't finished the course yet, but it is considerably harder. The depth of knowledge required is extremely high and there is certainly a big jump up from AS.
For AS AQA Biology, there is lots to learn and memorise. Virtually all of the entire AS papers are based on recalling information or applying it to vaguely different situations. In my opinion, the best way to excel is to simply practice numerous past papers. At A2, I don't think that the step-up is quite as bad as compared with Chemistry, it's just lots to memorise.
I think Chemistry and Biology would be good compliments to History and English Literature. I also chose 2 sciences/2 arts - I think it shows you have a broad range of abilities. Good luck.