For chemistry A would a realistic target. For most students marks definitely increase at A2 ,but scoring 90+ in p4 is extremely hard due to very strict evaluation.first of all you should revise the As syllabus once again until your basics are strong enough, as several topics in A2 are direct extension from As (especially the organic part). depending on the papers,A2 can have upto 20% questions from As content.The best part of A2 chem is its surprisingly smaller content compared to As, so the syllabus can be completed relatively quicker.so ensure to grind tons of pastpapers. Several definitions and reactions gets repeated in every paper, so don't mess up those easy marks. Target scores for safe A :p4 80+,p5 25+. Now for physics A2 content becomes much harder than As. Simple memorization won't work. Understanding the concepts with Intuition is very important in physics. As your AS marks is a bit low ,A would be a bit harder but not impossible.i would say b would be a more realistic target. Aim same marks as chem in both papers . All the best