As soon as you press RB or LB to choose from Light side or dark side ending the achievement will pop, right after it pops hit the guide button and exit out to dash. When you reload your game you have to do the last 30 seconds of the vader fight...most of which is a button time event...when prompted, choose the opposite ending and the 2nd achievement will pop!
*Please note that in the cinematic section will only have the ending that you watch entirely (or until it saves) so choose the ending you would like to keep last. (unless you plan to playthrough right away)