Werewolf: The Apocalypse - Earthblood

Either ending you choose you can reload the game and just restart the mission for the other ending.
Or you can copy the saved game on an external hard drive before you choose. Just remember to unplug the external hard drive before you continue.

This is not missable -- as per TA's definition, at least.

Forget about messing with save files and simply play the final chapter, make your choice, watch the ending and see the credits roll.

After the credits, go back to the main menu, select 'Play', highlight the save slot in which you played the story (should have 100% if you've done all side quests and gathered all collectibles), and then select 'Restart'.

This will put you in the very beginning of the last chapter. Replay it -- should take 15~20 minutes on easy mode -- and make the other choice to see the alternate ending and get the other achievement.

Simple and riskless.