Genesis Noir

You're don't need chapter select guys. After you finished the game, go to continue, and choose the other option. It's easier and it worked for me. You have to play just 2 minutes again till achievement.

*Missable achievement*

This is for going in the door at the end of the game. Soon after the achievement popped I quit out to dashboard and relaunched the game to get the other ending.

If you want to see both endings you can select the last chapter in chapter select. Either way should work

The game is really buggy, if you feel stuck or want a full video walkthrough use the video below. Full credit goes to Brain5454

Fastest method to get this achievement is by using the Debug Menu. From the Main Menu press the button to get into the Debug Menu:

Select Level
ENDING
Start At
ALONETIMELINE

Soon as the cut-scene starts playing the achievement should pop!

@ 0:31

Here is my 100% Achievement guide including ALL DLC! :)
Every achievement is story related, it is just dam weird though with some puzzles

As for the two endings, you can finish the game, hit continue and replay the last small section to choose the other door.
Takes nothing more than a few minutes, than you can enjoy your 1k and begin to think what in the hellsack did you just play?

Timestamps below :)


Intro - 00:00
Tutorial - 2:40
1. Chapter 1 (Seeding) - 10:35
2. Chapter 2 (Starstruck) - 25:50
3. Chapter 3 (Refelction) - 32:20
4. Astronomers Cut DLC (Chapter 1: Surveil) - 46:50
5. Chapter 4 (Hunt) - 51:05
6. Astronomers Cut DLC (Chapter 2: Observe) - 58:20
7. Chapter 5 (Gather) - 1:10:30
8. Chapter 6 (Thaw) - 1:17:15
9. Chapter 7 (Improvisations) - 1:31:10
10. Chapter 8 (Collision) - 1:41:10
11. Chapter 9 (Orbiting) - 1:59:55
12. Chapter 10 (Voyage) - 2:05:10
13. Astronomers Cut DLC (Chapter 3: Reveal) - 2:09:05
14. Chapter 11 (Singularity) - 2:14:05
15. Free Will - 2:48:00
16. Exodus Ending 1 (Not Alone Achievement) - 2:49:45
17. Exodus Ending 2 (Alone Achievement) - 2:53:30