It is possible to shoot a second layer of powder, but it needs to be done with more care than the first coat. I would say if you plan to shoot a glossy solid tone color, you will at least need to sand out the texture in the white power. If you are shooting a tinted top coat then you can pretty much shoot right over it. You can also shoot right over it if it's with another textured powder. It is actually possible to shoot up to 3 layers of powder on a single part and some times it's required to achieve a certain look. However this is always done with a translucent powder. So you could do a chrome powder base, with a tinted green metallic and a clear on top which would give you something like this
Doing one of our cases like this though would be super expensive, and it might not look exactly the same on every part. So the bay covers might be darker or lighter than the front cover.