Wonder how it would work? Duel wielding I mean, Dark and Holy elements contradict each other it is like putting the same charges of two Magnets together. They just don't get along, as for Restia.
Also, both Restia and Est uses the same kind of abilities, if you look at the illustrations of vol 5 with Est fighting and the illustration of vol 7 with Restia fighting both in their human form. Both uses their fingers or hands as a weapon.