Kagamime Rin & Len were born conjoined, and they were beta-tested as Danny and Demi in The Outfoxies.
The "petals" on Rin's headband are an integral part of her body. They contain a secret data-transmission mechanism based on ultraviolet reflectance and/or radio waves.
Megurine Luka is the Vocaloid universe's counterpart to Suzanne Vega. Luka used to volunteer as a waiter at Tom's Diner but quit after her parents became dissatisfied with her service and started abusing her (as mentioned in the song "Luka"). Len is currently the head waiter; Luka and Miku are frequent customers. Rin often stands outside looking in (at Len?), holding something similar to a Fire Flower(?), but why she doesn't actually dine there is unknown.