Story related, you will either unlock this or
after making a choice at the end of a mission. You can manually save when presented with the choice, then reload immediately after the achievement unlocks. Reloading will take you into the open world where you'll need to reselect the mission, then proceed until you can make the other choice. If you haven't yet started the mission, you may want to visit the Saints HQ before doing so, to set your spawn point for the reload.
Note: After completing the mission, you'll immediately
be dropped into a new mission. It is safe to save and reload at this time.