I did AS and A2 in one year. It's a lot of work and is inadvisable - if you have some sort of mitigating circumstances that explain why you did badly, it would be much better to do it across 2 years than one.
If you're intent on doing it, realise that you're going to have to do a lot of work each day, and that the volume of information you're taking in won't stick unless you find ways of making it do so.
Here was what I did: I needed to do the first module of A2 and both modules of AS (where possible) in the winter, so I finished the AS modules as quickly as possible so that the A2 information made sense (i.e, you need the foundation). Once I felt I knew all of the information for the modules (took about a month to do Chemistry, Biology, Maths and English AS working about 8-10 hours a day), I would do a past paper a week in order to retain the knowledge in my head. I also made a book for each subject with about 200-250 questions on them for the subjects where it was important to know facts rather than be able to just work them out (i.e, Maths). To do these questions, I used the syllabus from the appropriate website. This was especially useful for Biology and surprisingly useful for Chemistry, where you often need to know lots of little formulas etc. Leading up to the exams, I would have my Dad test me on about 50 random questions a day and I would also do a past paper each day as well.
About a week or 2 before the exam, I went back and made crib notes on all of the notes I had made before, essentially re-learning. Combined with doing past papers and the questions I had made, it created triggers in my mind that allowed for me to draw on the larger volume of information locked up in my head somewhere. All the while, I would be doing the A2 module "as normal", as though I were in the second year of college. It was kind of useful doing the ASs at the time as synoptic elements of the course jumped out when I was doing the papers, and even when it wasn't synoptic, you could use logical thinking to progress using both AS and A2 knowledge.
I think I did roughly 10-12 exams in the winter and 10-12 in the summer. It's been a while.
There's no denying it's a lot of work, but if you're determined, it's very doable.
EDIT: Also, I dunno what level of coursework you might have to do, but I arranged to do my AS EMPAs and Chemistry practicals in November along with resit students. Another reason it's essential to get the AS stuff out of the way as soon as possible.