Futakoi Alternative lead me to believe that biting the head of your identical twin was a sign of affection in Japan since Souju seems to be enjoying her sister’s antics (left pic).
Recently, however, I noticed a similar behavior in Galaxy Angel TV: Season 4 and these two aren’t identical twins. Given the serious nature of this show, I am led to two conclusions: 1) this is relatively common behavior in Japan since anime is an accurate portrayal of Japanese culture or 2) Mary and Volcott are, in fact, identical twins. The first one seems more likely since those two don’t look alike at all, but the Lost Technology that they encounter usually has strange effects…
Before I start making any wild speculations about this topic, you should check out the latest spatula and the semi-popular Pic of the Week.