Kagamine Rin and Len are japanese computer programs. The programs are also called Vocaloid (=vocal android). The reason they're always mentioned together? They were sold in one box. They were both voiced by the seiyuu (synchron speaker) Shimoda Asami. Some think they're twins, others say they're lovers. There's no real information so you can imagine them as whatever you want for your song. By using the program, you can create songs. They're great for duets.
Note: Since the names are japanese, Rin is the given name and Kagamine the family name. Same goes for Len, of course.