I noticed something neat. Ame and Aki do sound very similar to each other when singing. I first noticed it after a re-listen to "Suspect",
https://youtu.be/J1ObF5QF4o4?t=43.(timestamped for the part 0.44 - 0.55)
I can't tell if it's natural, or if they just have the same pitch correction settings - but they share a similar nasal-y/squeaky tone when hitting certain notes. Cute!