F7 and F9 are both numerical papers, in the sense that there is quite a bit of calculation. And they are both challenging papers and take considerable preparation. If you would like to have something lighter, you can combine either of those with F8. F8 is more written and doesn't take as much preparation time as the other two. So you could have a lighter and a more difficult paper, say F8 and F9, allowing you to dedicate more time on F9.
Having said that, many students take F7 and F9 together and do pretty well.
Hope this helps.
I would not advice to take F7 and F9 together. Both papers are very technical.
I had taken both F7 and F8 together and passed on my first attempt. This would be better combination.
I took F7 and F9 in September and it was very challenging preparing for both. I squeezed past F7 and failed F9. It really depends how much time, how resilient you are and how quick you learn technical things. Me personally I was jaded after my F7 exam and found it difficult to take F9. I am with the others it will be better to combine one of them with F8. F8 is quite straightforward.
Hope this helps! Best of luck!