You might say how was noble six with cortana on reach wasn’t she with master chief here’s what happened
the one with halsey then six
On August 3rd, 2552, Dr. Halsey separated Cortana into two “fragments”. One of these fragments, which contained all of her higher functions and personality, was stationed on board the UNSC Pillar of Autumn, while the other was tasked with mining data from the Forerunner artifact beneath Sword Base. When the Covenant began to glass the planet, Halsey tasked Noble Team with bringing Cortana’s fragment to the Pillar of Autumn. Although every member of Noble tasked with the delivery was killed in the process, Cortana was successfully brought to the ship. Afterwards, the two halves were reunited, and Cortana was able to use the data collected from the artifact to create a set of Slipspace coordinates which ultimately brought the Autumn to Halo.
so one fragment with halsey the other with master chief.
the one with the master chief
The surface of Reach is also overrun. The Spartans who John sent to the surface are presumably overrun and all slaughtered. Without the generators, the Super MACs fall silent and are destroyed by the Covenant, who soon begin glassing the planet. The remaining UNSC ships are all destroyed, except for the Pillar of Autumn, as Cortana plotted an exit vector in compliance with the Cole Protocol, though not completely at random. A comment by the Master Chief leads Cortana to reinterpret the alien symbols from the artifact as star charts. The NAV coordinates led the Pillar of Autumn to what the Covenant had been searching for: Halo, an extremely sacred artifact to the Covenant. The book ends with Captain Keyes looking through view screens at the ring, asking Cortana to identify the object.
you might also ask how is she capable of doing this here’s how
One of Cortana’s most unique abilities is her ability to create “clones” of herself, which she learned from a Covenant AI she captured and dissected. She can create multiple runtimes of herself to carry out different tasks simultaneously, and can control the level of intelligence and the number of abilities of each clone. For example, Cortana once created a clone of herself that contained copies of her decryption and translation software and set it to the task of interfacing with a Forerunner data terminal on Reach. Later, she was reunited with this copy, and assimilated the information that it had gathered.
so there it is how cortana was split in two one with halsey then six and the other with the master chief and then put back together on the pillar of auttum