Many actors train to be able to do actions ambidextrously. For instance, there is often two-handed dagger-twirling in superhero films, and I doubt all of the actors are naturally ambidextrous!
Evangeline Lilly learning to twirl daggers as Tauriel:
In
Thor: Ragnarok, Thor, Loki, Hela, Valkyrie, Hulk (who is CGI), and Executioner all wield two weapons at once at various times throughout the film. I do not know whether the actors in question are naturally right-handed or left-handed.
Example fight scene:
But yes, 'ability to train to do everything left-handed' is part of Maedhros' actor's job description, that's for sure! But since we typically don't know whether an actor is left-handed or right-handed, I don't think it necessary to make natural left-handedness a requirement.
FYI, Sam Heughan's character Jaime Fraser on Outlander is left-handed. Therefore, fans have asked the actor if he too is left-handed, and he said "No." Love story aside, I feel there's a lot of overlap between the character of Jaime Fraser and Maedhros. The left-handed thing just adds to that.
A fan page I found lists Henry Cavill as right-handed as well.
While injured, Armie Hammer signed autographs left-handed and joked that he was (sorta) ambidextrous: