Some does are better mothers than others, or are just genetically programmed to produce more twins.
But in a very healthy deer herd, ALL does should produce twins (they are designed that way--they have a biforcated uterus, meaning it has two sides, intended for two fetuses).
Now what to shoot comes down to why you are shooting does. If you are shooting does to control population, you would probably want to shoot the most productive fawn producers (the fastest way to reduce future population growth). But if you are just shooting does because they are a harvest opportunity and checking population growth is not a consideration (or population growth is desireable), then you should target adult does with the least fawns.