I had no idea this was possible until I did it last night. Marked three guys and pressed Y for the insta-kill. One of said guys was right in front of me, facing the opposite direction. Sam shoots the guy's two friends, and proceeds to melee kill the guy closest to me in slow mo. And yes, doing this gave me another execute.
This is probably old news, but I was pretty excited about the whole thing. Mind. Blown.