I am using a custom field with a special entry - so all your references you want to have in part A of the bibliography, I have chosen nothing (so it appears before anything else), but it could be something like “aa” and for all the references I want to have in part B of the bibliography I have chosen “mm”. In “Sorting” you just make a custom sorting, chose your custom field as first criteria (then - as usual - author, year, title or whatever you like) - that’s it.Â
At the end you have to insert your special heading before the first entry of part B.