It depends how your school/college organises it really. Personally I'd say taking AS in January and A2 in June would be a much better idea because then you'd have a chance to ease into the subject and develop your skills before going onto the more difficult part of the course. Trying to do AS and A2 at once could be quite tricky because often you need slightly different skills for each and you'd have to juggle them. I don't know about English language, but I think it would be possible with sociology, especially if you did the coursework. AS is planning a research project and A2 is actually carrying it out, so there's a big overlap between the two and it would probably be helpful doing A2 with AS still fresh in your mind. As far as the exams go, the style is slightly different at A2 with 1 long essay instead of 2 shorter ones, but the data response questions are pretty similar. I can only speak for AQA though, other exam boards may be slightly different.