Open After Effects, and create a new composition, with the width of: 614px, and the height of your artwork, in my case, 700px.

Now, import the background of you artwork into After Effects, and align it in the composition.
Leave an 8px gap between the side, and the main part of the artwork.



You should end up with something like this.

Now, add your animation. It can be anything you want. In this example, I will add particles.

Now, we're going to split the artwork into 2 pieces.

Pre-compose all the components of your artwork.

After pre-composing, make a new composition, with the width of 506px, and the height of your art, which is 700px in my case.


Insert the composition with your artwork into the new one, so the side isn't visible. It should look somewhat like that:


Now make another composition, with the width of 100px, and the height of your artwork(In this example 700px)


In this composition, you will insert the composition with you artwork as well, however, it will show the side only.

Now, add both compositions to the render queue, and export with your settings.