Assuming I'm understanding your question correctly, you want to be using two (or more) AI packs, one with a GetRandomPercent condition above another without it. With the first pack, they would mine rock A, else they would mine rock B.
Most of the chapel residents are set up this way - try looking at somebody in the Chorrol chapel for an example. Instead of wander packs, you might want to create use item at or travel packs instead.