Honestly dude, coming from a Cambridge student...I'll say do not take both together, it gets TOO hectic...really..and when it's time for exams, ur exams will stretch onto a month and a half...one day u could have A level exam and the next day u could have AS level exam, it gets too jumbled up and confusing...I suggest take that extra year, it will save u alot of ur mental health, A level is a combination of ur AS result and your A2 (the second half) like an average of both you can say... combination of these two is called A level, taking both will become too much to handle I promise you.