According to the docs on this page:
The following "multi-dimensional index" should be perfectly possible, with a default hydration mode:
SELECT b as business, p as product FROM Businesses b INDEX BY b.id JOIN Products p WITH b.id = p.businessid INDEX BY p.id
However, b.id is completely ignored (it is a numeric primary key).
I tried to go further, giving 2 products a matching barcode and indexing by barcode and then a (unique, numeric) productid. Only the barcode worked as a key and only one of the products with a matching barcode was selected. I used this query to test:
SELECT p FROM Products p INDEX BY p.barcode JOIN p.businessid b INDEX BY p.id
I also flagged the docs, because I don't think a userid should/could be starting from 0.