Salaam angel,
I hope u understand that the jump from IGCSEs/GCSEs to AS is a very big one and so is one from AS to A2 but not as big a jump as that from IGCSEs/GCSEs to AS. We do not have to do the mistake of rushing in such an important part of our academic life. Skills can't b crammed for but they r gradually built up. I hope u understand wut i mean here.
I know it will look good in ur medical application (u r applying for medicine, ryt?) that u hav done all ur 4 A levels in a single year (this is wut u r planning to do, ryt?) but they won't really decide whether to give u a place in medical school on this basis. Atleast this is wut i know.
My suggestions: Don't pressurise urself. Finish ur AS units first because the A2 units r synoptic. Synoptic means u will need to use knowledge from different areas of ur A level course to answer the questions asked. First take the AS units with ur A level board and see wut u hav to do differently the next time. The A2 units r also a bit more advanced than the AS ones and the topics in the A2 units require a much deeper understanding than those in the AS ones. After all, the A level grades r used by institutes worldwide to decide who to give a place and who to not and ofcourse many other things matter as well but wut i mean is that u can't take risks here, can u dear? I strongly suggest u to start with AS at school, see how it is and ofcourse study at home as well. Divide ur AS units between the January '12 and June '12 exam sessions. But do remember, do not underestimate A levels. If while doing AS at school u think u can also do the A2 units, u can always start studying for A2 at home and take the exams if u wish.
In the Edexcel A2 Biology units this year, we were also tested on different areas of the AS course. Indeed the A2 topics build up from the AS ones. Haven't u noticed in the Edexcel A2 Biology textbook how the different topics r built up from the AS topics?
But everything is possible if u r willing to work hard and achieve ur goals successfully. On the other hand, studies aren't supposed to make things difficult for us. We study so that we become better humans and live happily. I don't want to sound like a granny but atleast these r the suggestions that i wud giv u as a sister.
If u wud like to ask anything, u r always more than welcome to ask. Hope it helps dear. Take care and Good Luck.