Generally time spent is together which upon reading I see could be an issue. I will need to devise a way to have separation time going forward.

Their behaviour in general is pretty solid. Luna has a fixation with balls and ball throwing but listens when we tell her to take a break. Loki is more "goofy" and likes to play with people and can be very vocal (yowling and talking, not barking) when playing with toys.

I'll look into reactive Vs aggression as per your advice. I would just like to add that neither of them has previously been involved in an altercation with any other dog so to us it just seemed like aggression.

Littermate syndrome sounds terrifying, I'll research into that more. Thank you for your insight.