Here is the Errata on Flurry:So there is no reason why you could not flurry a wall and then the character behind the wall. As long as they are adjacent to you at the time of the attack, it's legal.

I was trying to think of an instance where you would be knocked back between flurry attacks but I couldn't. (I don't know it that can happen.) Because if you were and you ended up next to another fig, you could use your second attack on them.