Posted on 2 February, Japan's "Twintail Day" (ツインテールの日 Tsuintēru no Hi), when (according to Japanese Wikipedia) a man gives the girl he like two hair-ties and she accepts by putting her hair up in twintails.
Proposition 1: "twintails" in proper Japanese is 二つ結い futatsu-yui, and "Yui" happens to be a real Japanese surname; name these girls the "Yui Twins" (futatsu-Yui).
Proposition 2: for individual names, "Sae" (サエ) for the one with the hair-tie on her left (左 SA, hidari; therefore, "Lefty-chan"), "Ue" (ウエ) for one with the hair-tie on her right (右 U, migi; therefore, "Righty-chan"). An added pun could be that since "ue" (うえ) also means 上 and can be used to address a superior, Ue-chan is the second-born.
