If you have multiple objects you are mixing, I'd take the RGB approach to isolate where they are, and have natural distribution rather than funny artistic waves and patching. A RGB output to the distribution shader with seed and noise settings would be awesome to make setup easier, and is a far more natural approach to something like vegetation distribution like under the hood of other software.
https://www.facebook.com/groups/Terragen.Galleries/permalink/2368238406635502/Note: When setting up any population mixing with others (inverted PFs or not) it's good to pay close attention to the object spacing. Try to keep it about as large as the model itself, or just under. Also, you may want to use breakup, as it causes strange effects to the noise, I'm assuming it's effect is unclamped or something.
Also in Kevin's example tutorial, it's strange we go from varied populations to non-varied populations, and back, without explanation... previews are suppose to provide the result the user should roughly get.