User blog comment:B214/High School DxD New Episode 3 Discussion/@comment-16075832-20130722160729/@comment-99.1.221.23-20130723035642

I wouldn't factor the concept of who won the sparring match too much into the situation.

Both parties have a common goal, which is to destroy the Excaliburs in possession of the Fallen Angels. They have a much higher probability of fulfilling this goal by cooperating. Xenovia and Irina only have a 30% chance to survive and would obviously much rather walk away from the situation alive rather than dead. If Yuuto strikes out on his own from Rias, he will become a Stray Devil and will inevitably be hunted and killed.

By cooperating, Xenovia and Irina have more manpower and better probability to succeed. Issei and Koneko also have the chance to save and help their friend. It's possible to have conflicting motives and ideals and still cooperate to tackle a common foe or problem. Ddraig and Albion were actually sealed away when the Three Great Powers worked together to defeat them.