What will be the arrangement of crystals if a solution of two salts, say, Sodium Chloride and Potassium Chromate, is dried? Will one salt crystallize before the other and become the crystallization nuclei of the other? Would both types of crystal form in a chaotic mixture? Or would they interact to form some new lattice?
As @Ivan Neretin states, it depends. For example, potassium sulfate and aluminum sulfate form "alum", with a single crystal structure.
Tartaric acid (and its salts) occur in different forms, and even though they have the same chemical makeup, these stereoisomers can be separated because they form differently shaped crystals. One of Louis Pasteur's first studies was separating the isomers from a mass of crystals.
A good source on growing crystals is Crystals and Crystal Growing by Alan Holden and Phylis Morrison