User blog comment:Atsuko-Regalia/Yumemi Nemu, Tone Rion V4, and Nemu Yumemi/@comment-5051846-20170114130525

''Isn't the point of Vocaloid to act as a replacement for a human singer, and provide different vocals to suit different needs and roles? Then why are you having two seperate Vocaloids be voiced by the same woman? ''

Tortoishel makes a good point, but I think there's another reason for this too! A Vocaloid is not as flexible as the human voice. Nemu's voice provider can make tones that Rion alone cannot. Therefore by providing two Vocaloids, it gives the producer a bigger range. Think appends!