User blog comment:B214/Light Novel Volume 16 spoilers/@comment-1359322-20140128102256/@comment-1359322-20140128110752

We don't know for sure, since they are the same transcendental being, they might understand the consequences should they fight, their battle might cause sever consequences like the Dimensional Gap being destroyed in the process which they both don't want.

Also Khaos Brigade might also played a part in making sure Ophis doesn't fight Great Red. If they didn't promise Ophis that they would help defeat Great Red, Ophis might have fought Great Red long ago which would indirectly cause Armageddon to the world.