It means you make one type of mating, usually selected by coloring attributes, for exhibition quality males, and a different type of mating for exhibition females of the same variety. While color would be the primary reason for doing so, it could also be done for certain type attributes. The multicolored varieties would be the most logical candidates. As one example, wheaten or blue wheaten males with hackle striping might produce better colored females than males without the hackle striping. It sort of depends on the line you are working with as to whether double mating is actually needed or not. I suspect most of us don't bother.