Just a mashup of "Lullaby for a Princess" by ponyphonic and "Luna's Reply" by MsIkarishipper.
Picture is by Zedrin on deviantart.
I own nothing.
UPD8:
Huh, I honestly didn't think that this would gain popularity so suddenly.
The headphone deal was born from necessity, although it created a neat effect. When I tried to do it normally, the two parts were hard to make out. I tried to balance the two halves, but it didn't appear to succeed quite as well as I hoped.
Anyway, some have been asking for a download, so:
http://www.mediafire.com/download/6dbkg5f1ty1w0t2/Lullaby_Duet.mp3