The dark blue one is luna, the sister of the White, who's name is Celestia. Celestia is princess of the sun and luna is the princess of the moon. Everyone loved celestial sun and had fun in it every day, but everyone slept through lunas night. Luna thought she wasn't aprecciated, so she transformed into an evil mare of darkness, nightmare moon. She tried to cast the world into eternal night. Because of her crimes, Celestia,regretfully, banished her sister to the moon for a thousand years because of her crimes. Here, Luna and Celestia are battling because Celestia is trying to baish Luna and Luna, obviously, doesn't want that.
"The stars are the keys to the gate, whereby the spheres meet. Ponies rules now where She ruled once; She shall soon rule where ponies rules now. After summer is winter, after winter summer. She waits patient and potent, for here shall she reign again."
--- Final ramblings of earthpony Shade the Shadow-Priest, Cult of Nightmare Moon, before her execution. Year 687 Celestian Era.
I think I got it. It's because they move in opposite directions. They both seem to be attacking, but position of wings shows more defencive pose. This factors compensate each other and whole motion stops. And maybe there is something you could do with Tia's mane. Some small details that would show that mane is not solid. I hope I'm not picking on too much. I like when people say something else except "cooooooll" and "daaaaaaaw" about my works, and that's what I'm trying to do)