The other pair is from Kyoshiro To Towa no Sora. Chikane and Himeko are from KNM, Kaon and Himiko are from KTTNS. They're also feature in two other series. People say it's different incarnations of their lives.
This is something that makes me think "What happened here?". I mean Chikane (dark hair) actually looks a little angry when she's doing the spanking. Himeko (blond) looks like she wants to run away. Sexy and funny.
It's Chikane and Himeko AND Kaon and Himiko... That's why there are four of them. On the left is Chikane and Himeko, which is why Himeko is all lovely eyed. And on the right is Kaon and Himiko, which is why Kaon really likes it and Himiko is blushing 'cause she does too. xDD
Just FYI for all the people who may care, Himeko and Chikane are the two main characters in the shōjo ai anime OVA (12 eps) and manga (2 Vols) known as:
"Kannaduki no Miko" or "Kannazuki no Miko" (Priestesses of the Godless Month)
You can find a Wikipedia entry here:
I loved both the opening theme ("Re-sublimity" by KOTOKO, a house number with a dark driving beat) and the ending theme ("Agony" by KOTOKO, more love ballad.)