Is f6 and f9 good combination to sit in one attempt and pass?

At this point assuming you will be studying these papers from now up until the examination date in December this is a bad idea as it may well see you failing both. Reason being they –

1. Both are huge syllabuses and need at least 3 months studying each with constant practice
2. Are unrelated papers with the exception of the tax treatment of the NPV calculation for tax allowances in paper FM
3. Both are new subjects as there is now previous related paper as we have seen with MA > PM or FA > FR

This is just my opinion so you may succeed but both are demanding papers. Good luck

If you now plan to sit one in December and one in March, the preferable order is TX, then FM.
This is because if you were to be unsuccessful in TX, you would be able to resit it in March under the same tax legislation. The syllabus “year” is Sept-Jun/Aug for all exams except TX/ATX which is from June to the following March. So if you were to sit TX for the first time in March 2021 and have to resit, the Finance Act (for example) will be updated.

You should plan to do LW – assuming ENG or GLO you can sit on demand in later Dec/early Jan (say), or earlier if you are sufficiently ready. If you plan to sit two applied skills exams in December session you will be constrained to the same week.