I thought this was obvious speculation fuel as well (though I didn't add it to the pool).
However I just noticed something very obvious that would lead to them both making those expressions.
If you look in the center of the picture you'll see that their hair braids have become intertwined. Sakuya is slightly annoyed by it whereas Meiling is very happy about it.
I'm sure a romantic theme began playing in the background the moment this happened.
lastsinz said: But now we speculate on how did this event started
Easy, they saw Keine headbutt one to many times and wanted to try it out themselves at some point. Headbutt by Sakuya cause she's the one that likes to hurt and she saw an opening to try it, braids flap around during headbutt twisting around each other they pull away and the braids tighten... ONLY FLAN CAN HEADBUTT LIKE KEINE!
Sakuya caught a case of the slackies from Meiling and fell a sleep right beside her. Marisa then walks by and ties their braids together as a joke and then heads off to "borrow" some books from Patchy. Later Sakuya wakes up and we have our current situation.