To get this you must finish the game with a partnership camaraderie level of "Ambiguous ...". You can check this by hitting the START button and looking under "Partnership" in the bottom left corner. As long as at the end of chapter 7, right before you plant the C4 and enter the room, your partnership status reads "Ambiguous ..." then you will get the achievement. There will be some random word after ambiguous but that doesn't matter.
When not in combat, go up to your partner and hit to give him props, this will increase your camaraderie. Stand outside the final C4 door in chapter 7 and hit over and over again until your status is correct.
To get both of these in 1 playthrough
After you open the final door by placing C4 on it, you will be asked to make one last decision. Make your decision, watch the cut scene and now the mission review will appear. After you hit A on the mission review screen press and reload last checkpoint. You will have to do the battle on the stairs again. Once that is complete just keep tapping or near your partner to change your camaraderie in the opposite direction.