I personally have not tried doing both AS and A2 in one year, but my college offers it for some courses to some people that did well enough in their GCSEs, however I have never heard someone say this is easy. This year I've known two people who took two A2 and then did citizenship AS and A2 this year, and it's certainly a risk. You don't have the indicator of an AS result to know how you might fare so you're going in blind. If you're talking about doing 3 A2 and then having an additional AS and A2 of maths along with it...I would strongly advise you don't. Not only do you still have the uncertainty of the previous scenario but now you're technically doing 5 subjects and some subjects build on what you've learnt in AS in A2 so you might be doing an A2 topic that requires some basic previous knowledge that you don't have because it's two more months before you cover it in your AS. I did four full A levels this year and it was difficult to say the least - you need to have good time management, be willing to sacrifice a lot of personal time you would spend with friends etc, and you need to be sure that you're not damaging your results because you're stretching yourself thin. Overall I wish I'd only done 3, it was of absolutely no benefit to me to do 4. Of course this is just my experience so take it with a pinch of salt as I'm sure there's someone out there with a different experience to me.
I'd definitely talk this over with your teachers who can assess your situation with a better knowledge than any of us - they'll know your strengths and weaknesses and how driven you are to see it through.