Isn't your query the same as the following:
SELECT UR1.Name, ...
FROM UserRole UR1
INNER JOIN UserRole UR2
ON UR1.RoleID = UR2.RoleID
WHERE UR1.UserId = 2
AND UR2.UserId = 6
You will have to add the relation in the RelationPredicateBucket Relations collection
And add your filters to the Filters collection in the same object.
And then pass your bucket to the fecth method.