Hand of Fate

1: When you knock an enemy down you preform 2 slash attacks to the downed opponent, the 3rd is a downward strike that kills an enemy in one strike (if it lands. Often if an enemy still has most of it's health it will roll out of the way, also you can never do it on a boss no matter the health they will always roll) you need to have enough speed (Items or high combo) to preform that 3rd attack on 2 enemies you knocked down.

2: An easy way to obtain. equipping the Blood Crescent sword with the Enormous Shield.
if you bash any normal enemy in the game with Enormous Shield it knocks them down on the first bash, do this to 2 enemies then with the Blood swords finisher bonus you do the finish strike on the knocked down enemy without the 2 slashes first, instantly killing them.

I used this combo all the time running this game. As my only sword and shield in my deck with them both combined you're pretty unstoppable killing everything but bosses in 2 hits but also the Blood Swords weapon ability is a instant kill on ANYTHING including King of Scales for a small exchange of around 10% health. Use it wisely on enemies you know will hit you for more than that and its nothing.

For this achievement I used the blood crescent weapon, this weapons allows you to kill enemies in one hit once they are knocked down, so obtain this weapon then find a combat card and knock two enemies down to kill them both in a row.

The blood crescent is unlocked by beating the King of scales, which is the final boss in the game not including the dealer.

Afters weeks and weeks of attempts I finally got this today.

In the past there have been times when I was certain that I killed two prone enemies but it never popped anyway this is what I did.

First of all I unequipped my shield, then I managed to purchased the Blood Crescent. I had followed the guides before using this sword but for some reason having no shield equipped makes putting the enemies down is easier, then the sword finishes them off in one move. The enemies were lying right next to each other and I was able to kill them both before one of the got up.

I tried the method with the enormous shield and as soon as I would knock an enemy down the other would get back up. So If you have been struggling with this achievement try unequipping your shield. I got it in my first fight.