Okay so I just got this and it's easier than it sounds as you DO NOT have to get both kills in a single game, as I just got in an encounter battle, sat in the flag and got 1 kill but died. Played a second match and again luckily I was put in an encounter battle, I rushed to the flag and killed someone who was on low health and as I soon as I did the cheevo popped! :D
So to clarify;
You DON'T have to get two kills while in a flag in the same game.
You DON'T have to win the match.
You DON'T have to survive.
It CAN be done in an encounter battle.