Fake parallax effect with 2D layers and a null?

Here's a simple example to give you an idea of how you could do it with expressions. Add a null to your comp (called "Null 1" in this example) which will act as the motion leader for the other layers, but don't parent your 2D layers to it. To each of the 2D layers, add a Slider Control named "Parallax" and set its value to what percent (0 - 100) of the null's motion you want the layer to receive. Then add this position expression to each 2D layer:

center = [thisComp.width,thisComp.height]/2;
leader = thisComp.layer("Null 1");
offset = leader.position - center;
parallax = effect("Parallax")("Slider");
value + offset*parallax/100

Each 2D layer will receive the appropriate percentage of the null's movement away from the center of the comp (the null is assumed to begin at the center of the comp). There are lots of ways to modify/enhance this, but it might be a good place to start.

I recorded this tutorial in my character animation tutorial series showing how to create such a 2D parallax background in After Effects with my (paid) extension iExpressions:

To create a paralax effect,  the key is that the layers don't just have a fixed offset: The more far away the objects are, the less they need to move.