What do combined science grades mean?

So, you take 6 papers in AQA on all 3 sciences and get a combined 2 grades. What contributes towards each grade?

All papers count. Each papers is worth 70 marks, so across the 6 papers you can earn up to 420 marks.
They then apply grade boundaries, as with any other subject, but your single grade happens to contain two numbers rather than one. It's still a single grade - the two numbers don't mean anything individually.
They use two numbers rather than one to indicate that the exam is the size of two GCSEs, and so is worth two GCSEs. It also allows them to (sort of) create "half" grades. In a standard GCSE you might get a grade 6 or a grade 7, and in a double-award GCSE like AQA's Combined Science you might get a 66 or a 77. But you can also get a 76, which is half-way between the two.
Note that the two numbers are always the same or are one different by one. So 66. 76, and 77 are all valid grades - but 75 is not.

Thank you