So right now all we can do is make a sort by use an assigned filter by value for sorting like this:

_eval([ (brand:Nike):3, (brand:Adidas):2 ]):desc

With the above example, the filters are evaluated one by one, and when the document matches a given filter, we stop and assign that value to that document and this value is used for sorting. We don't have a way to combine and add the values like you've described.