I think it's doable if the subjects overlap, as quite a bit of the material for some Access subjects is similar to AS Level. I used AS and A2 textbooks quite a bit on my course. It might actually be helpful to do the two together, but it will undoubtedly increase your workload. I wouldn't recommend it if you have other commitments too.
If you decide to go ahead, make sure your Access tutors know, and if you tell them the date of the exam well in advance, they will hopefully try to avoid scheduling Access exams at the same time.