Dragon’s Breath’s AoE component never states that the creatures in the cone are targets. The Fireball spell does however state that creatures in it’s area are targets. RAW I feel like the first spell should be Twinnable. Dragon’s breath is an exceptional spell that involves a single-target choice followed by the creation of an AoE with targets of its own.
— Jeremy Crawford (@JeremyECrawford) December 23, 2017