Ah, memories… my brother and me, our parents’ mirrored closets, and the “hall of mirrors” we set up to travel to other dimensions…

Did you notice? We’re already not showing Frigg and Gravedust in the same frame as the other three, after the first shot. The trigger for the spell is simple: if you and your friend look away from each other for more than a second, they become invisible to you and you to them. The others are looking at Best and he sees them in the mirror, so they’re still together. Gravedust’s eyes never quite leave Frigg even as he notices the door, so those two stay together. And as soon as everyone’s eyes leave the door…

Drawing reflections, incidentally, is much harder than it looks.