Animation of back and front a part of a prefab individually on unity

I am attempting to animate a platform back and front.

As a result of, when my character will probably be on it, it need the entrance of the platform to be in entrance of the character.

Here’s what I’ve to date :

The factor is, now I need to animate it, and the little inexperienced half you can see on the left is shifting and have to come back in entrance of my character. BUT, when the inexperienced half is on the again then my character needs to be in entrance of it.

The order is that this :

backpart : order in layer : 9
character : order in layer : 10 
frontpart : order in layer : 11 

And, the backpart and frontart are collectively in a prefab like this :

- backpart 
   - frontpart (son of backpart)

I’ve no downside to animate the backpart however I can not discover any method to animate the frontpart WITH the backpart.

Within the animation window I can not put a second row of sprite and I do not know the opposite manner so I am kinda caught.