Brds are afraid that if it were to be added to Gjallarhorn, it would require finishing a Stage 2 lvl99 Gjallarhorn in order to get it. Which would be ridiculous... :/
2nd lv99 trial only adds the afterglow. They're not going to make a special song require that. In fact my understanding is they won't have a 2nx 99 trial because iirc, the horn can't have a glow.