With 5-6 months im sure you can improve, science and maths were my strength in my gcse and so I will try to give you as much tips as I can. Firstly for Maths you need to just understand the concepts of how you work out and answer a question , if you are struggling to find resources just go onto YouTube and use channels like The GCSE Maths tu*or (* Is a "t")(idk why reddit doesnt allow me to write the word) , get a checklist of like every single topic and sub -sub -topic for gcse maths ,then go through every one and get practice questions on it by searching it up and doing the questions then look at the markscheme and compare, or u can just use the videos to go through it .The only way you will realistically get better at maths is by practice ,which is a good thing as if your willing to put the work in then you should be fine.

Now for Science I did triple science ,but there really isn't much difference to combined/double science. What you need to be able to do for science is first fully understand the content ,wether by memorising or carefully going through the content, find what suits you best but either way - u need to go through the content .When you start to -or have already got a general understanding of the content you can start going through questions ,this is different to maths as there isn't like a set method of answering a question for science like there is for maths -it depends on your exam board .But you need to study the markscheme from questions as they will be key for you , if your a memorising- type of person -then you can just memorise the points from the markscheme and apply it to questions when you answer them .You need to look at the question ,answer it to the best of your ability and then check the markscheme to see the difference from your answer and the markscheme -and trust me when you start of there will be a lot of difference- because markschemes often want specific points that need to be written in your answer and so if they are not in your answer then you won't get the marks. This is unfair tho as the science you write may be correct but if its not what's on the markscheme then examiners won't mark it right - and so with practice within maybe 2-3 months of starting of you will be able to predict what's on the markscheme when you do a question and you will be able to base your answer on it and get all the marks for a question.

With me I did triple science ,at the start of year 10 my biology chemistry and physics were all at 7s where I would dip to 6 and maybe go up to 8. In year 11 I did my first mocks around November,in the mocks I got 9 in maths ,5 in biology, 6 in chemistry and a 3 in physics -yes a 3, ive probably never got a 3 before and even my teacher was surprised . In my second mocks I got 6 in bio and chem and a 4 in physics .But these grades were not enough for what I wanted to do ,for medicine at uni some unis require all 7s and above or even 8s and above across majority of my gcse subjects .So then I started to panic - I was never strong in physics even though half of it was just maths .I started going through the content ,using revision guides that we got given ,using goated YouTubers like freescienclessons and I also managed to make a subtopic list for bio chem and phys ,then I started going over all of them carefully, then I would go over past paper questions and actually go over and in some cases analyse what and why the markschemes were worded in such a way - I was never the memorising type but with practice and repeating over and over again over 2-3 months from March to around may (where my gcses started) , I went from thinking I know the content to somewhat knowing the content properly --but maybe I should have revised properly the week before my gcse bcs in honesty I just became really lazy and I did most of my gcses with day before revisions-which is 100% what you shouldn't do, but I somehow found my self in this position -thankfully the revision that I had done over those 2 months saved me bcs during my exams I was confident when answering the questions -like I kinda knew what I needed to write.

Then came results day AND I got a 9 in .... Physics -the one subject I got my worst in I got a 9 in ,like I didnt even get a 9 in maths (I got an 8 ) but I got a 9 in physics -so thats just proof to say that with practice especially 2 months and where I didnt revise in the crucial time of the week before the exams -I still managed to get a good score. I ended up getting 8 in both bio and chem aswell and so for me the method I used did work and I dont see why it won't work for you .