Equip the assassin hidden blade and approach two unaware enemies who are near to each other. Both of them should light up and you can assassinate them together by just hitting the button.
You can easily do it to two enemies you meet just before the boss to sequence two. Instead of heading straight towards them, skirt around by climbing the trees nearby. Eventually you should be able to hop onto a tree trunk right next to them and then just drop down and kill them together. --- Now playing Ragnorak Odyssey (R8), Project Diva and Wild Arms