How do I mix charcoal and graphite correctly?
Depending on the paper and pressure you apply to your pencil, when you use graphite over charcoal, you will just scratch away most of the charcoal powder. Fixating the charcoal first won't help much, since the charcoal particles are much larger, and this process will just mess up the drawing more.
On the other hand, using charcoal over graphite won't work well either, since the charcoal might not get a sufficient grip on the graphite wherever it has been applied heavily: using pencils applies pressure that flattens the paper, while the remaining texture will be filled up with graphite - the glossier the pencil marks, the harder it will be to apply charcoal over it.

You can combine both techniques, but will have to do some planning in advance so the two won't mix as much.
Instead of using regular pencils, you can use grey or coloured pencils, which will have a much more matte effect.
If you're using pencils because of the reversibility, and using mediums is out of the question, the only real option you have is to only draw very lightly, preferably with softer pencils (e.g. 9B).