Chase, Rescue
They're enemies.
No: they're rivals. Determined to make her choose between them, determined to not lose to each other.
But when she's in danger, Neville puts aside his rivalry for her sake, bursting into his classroom. "They're chasing her," he blurts, and he doesn't even say who: who is chasing, who is being chased. He doesn't need to.
Severus is out of his chair in an instant, wand grasped in his hand. And together the chasers become the chased, and when spells fly and splash against the walls, they stand back to back.
Luna clings to them both afterwards, trembling. "Don't make me choose," she whispers, "Please don't make me lose either of you."
They look at each other over her head, size each other up, then nod reluctantly. She has chosen, after all, and because she means everything to them both, they'll accept her choice.