Shatter only works in one direction so what you do is take your first image, animate the force to make it shatter, then apply the same settings to your second image. Precompose the layer you wish to Assemble then Time Reverse the layer and adjust the position of the pre-comp in the composition.
It's really simple. Let me know if you need more info.
Rick's correct, it's easy. The problem most Shatter users have when reversing the action is getting satisfactory ballistics. Shatter blows things up so the whole system is designed with its physics going in one direction and they get there really fast.
You can Time Reverse a precomped Shatter effect, you can even alter the curves on those keyframes, or you can render the movie and reverse it. CC Force Motion Blur can help Shatter's lack of rendering elegance.
Shatter's effects are almost totally repeatable using the various Seed fields to hold known values that you write down and enter manually. That means, with no small amount of effort, you can use the same fundamental settings to place all of the particles at precise locations in 3D space and you have total control to the initial physics parameters so your elements do not appear to explode but rather assemble.
When using Shatter as a blow--it-up-then-assemble-it engine, my challenge has always been to make a convincing or appropriate transition between the comps. You can add viscosity to the end of the explosion so your particles slow down to a halt, transition, and then ramp the speed up as the action reverses. One way is to have a version of your second graphic used as the backplane. So you need four images on two comps: original that blows up, a backplane of the assembly image; the assembly image and a backplane for that of the original blown up image.
It's not easy but it's a ton of fun and arriving at believable ballistics on the assembly totally sells the effect. Plus, you raise your comprehension of AE to a previously unknown elevation.