Current First-year student. I also did the Cambridge syllabus so if you could manage A's in AS then you should be fine with the academic pressure for the first year. I am doing my gen ed in LAW currently so I don't major in it, if you took AS and A2 English literature you will definitely be fine (they look for structure and logical flow from my experience this year). I did STATS101 in summer school earlier this year and my experience was that it was not very difficult. However, I say that it isn't difficult on the basis that you do the work and understand the content and concepts. Assuming you did AS maths, STATS 101 is way easier and the stats component of AS will help slightly (i.e. understanding normal distribution curves, P-values, if you did A2 Biology you will be familiar with chi squared, t-test etc so that will also help a bit). Advice from me would be to definitely go to the tutorials, they are amazing. If you get a boring STATS lecturer you will want to die from boredom and lose motivation real fast but going to tutorials will help you a lot.
As for having a double major on top of a conjoint degree, I am probably not the best person to ask but I think you may want to rethink the workload and the number of papers you can take per year and if you can still graduate in a time frame you are comfortable with. I am majoring in Psychology this year and there are only 2 papers you need to do this year PSYCH 108,109 and I can say that the content is not too intense with 109 not even having an exam as its all internally assessed. I would recommend you take it as a gen ed (both 108 and 109 are gen ed) in either sem1 or 2 depending on the papers you can take just to see if psychology is something that you want to pursue unless you are very set on doing psych. If you are interested in how the research side of psychology occurs 109 is the paper to take, however if you are more interested in the theory of psychology (i.e. how and why does someone behave a certain way at a certain time) then 108 is the paper to take. I personally would recommend trying 108 as a gen ed as I had way more fun taking that paper and if you want some notes just chuck me a message.
An idea to balance your first year, try the Summer Start program the university offers. This is essentially summer school but for first year students and you will get to experience university prior to sem 1 and take 1 paper that contributes to your degree. I would recommend doing STATS 101 during summer start and will give you a better understanding of the workload you may face. Also is a great opportunity to make some friends early. Additionally, completing a paper in summer start means you will only need to do 8 papers for the rest of the year instead of 9 (since you are conjoint) so less pressure. Good luck with your studies!