Edexcel sorts it out for you as follows:
- For Maths A-level, it will put C1-C4 and a legal combination of two applied modules which give you the highest grade with the lowest number of UMS points (if this makes sense to you). "Legal combination" means a "2" module can only be used only if the corresponding "1" module is used.
- The other 6 modules will go to your Further Maths.
For example, say if there is a combination involving C1-C4, M1 and M2 which comes to exactly 480 UMS, these will be used for your Maths, and the rest will go to Further.
This is what I've come to believe, anyway.