This is one of the eight different endings that is available to you in the game. This one is a bit tricky if you don't realize it early on.
Point #1: This is only obtainable through a 2:1 Heart Point playthrough. What that means is that you must give two heart points to one girl, and the last heart point to the opposite girl. So, by the time you face MYU at the Battle of Fate, you will have made the final choice and cannot reset this unless you have a prior save, or play New Game + and start at Phase 2.
Point #2: Do not enter a Level 8 Cosmosphere, at all.
Point #3: After clearing Level 2 of Tyria's mock Cosmosphere, never enter it again. EVER!
By the time you complete Phase 4 for a True Ending, you will receive this ending with a conversation between Cocona and Aoto on Ar Ciel.