Emit Random Particles?

According to @Eric5h5 in this post, you cannot have multiple materials on a particles. Given the age of the post, he was probably talking about the Legacy (ParticleEmitter) system, but it is likely true for the ParticleSystem as well. And it makes sense. To get the kinds of performance for the number of particles created, these systems likely drawn all the particles as quads in a single mesh, and I've seen nothing in the reference that gives access to this mesh.

On the other hand by jumping through some large hoops @JPLKit was able to change the image on individual particles. It required a custom shader.

http://answers.unity3d.com/questions/17246/how-to-make-a-particle-system-with-different-parti.html

The simplest way to acheive this will be to have 2 particle systems, each with a different particle material. Then just randomly choose which one to emit from.