How to auto reverse key frames

HI,
I am sure this would have been asked before. I searched without any success.
Does anyone know how to auto reverse say keyframes 1-10 ?

What I want is to play 1 -10 (tween) then from the 10th frame I want to reverse play the tween in a loop.

I only have two layers . Basicly object on the left goes right and object on the right comes to left . Simple animation bit like flip flop but I want it in reverse in order to have the smooth transsion.

Thanks in advance.