Drying the solution of two salts
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