i think it's Zatsune Miku

Shotacon....? 0w0

Well,I did some research,and I figured out that their supposedly neither twins nor a couple,but,as Iskatten said,mirror images of themselves.This being said,their last name actually gives a little of the same idea.Kagami meaning Mirror and Ne meaning Sound.Meaning what it says,Mirror Sound.
Although ones could argue about this concept, (many people do from what I could see) that's pretty much how it is.
The creators mainly just wanted to have a twin like sound between the two,thus creating the idea of them being twins.Although,they didn't really ever decide if they wanted them to be twins or a couple,so it was never established.From what I read,it said they wanted to keep a creative view for the fans.
So,I guess it could be either or for each of the fans.They decide what they want them to be,which is why it's different for each person.Some say a couple,others say twins,and some say that their mirror images.It all depends on the person basically.

Anyways,here is the main link I found that talks about this. ---> http://vocaloid.wikia.com/wiki/Kagamine_Rin/Len The most information about it is at the very top and somewhat middle of it.

Then I will take them as twins who have fallen in love with each other having a forbidden love.

. Gakupo & Luka <3

Len x Neru looks cute too

Luka and Gakupo.. still on top

. Luka & Gakupo all the wayyyyyyyyy :3

eh kaito and miku too X3

. LEN & MIKU <3 , idk why , hahha , i tried on partnering them in my psp [Vocaloid Hatsune Miku Project Diva 2nd] & and i think they kinda fit together