The real question for this species is the benefit in evolving with two vocal cords. If it speaks in unison like you portray, the common person should never be able to hear both. The major benefit would being able to cast two spells at the same time or even more capable of mimicking other voices and sounds. The unfortunate aspect to this is the creature would need two separate esophagus for the air in its lungs to travel through two tunnels or two mouths. I really like the concept and excited to see your idea unfold.
Agreed with adding in a once per long rest type use of casting two spells within the same round. I would lean towards the race being able to blend seamlessly into societies without them knowing that they are this certain race. Could be a twist on anything you want (elves, humans, dwarves, etc) Giving them that element of surprise. There could also be people in that race that are very open about it, wearing certain clothes, having tattoos, etc.