I tried messing around with dual brush and that doesn't work. The second/dual brush acts as a mask that is applied to the top/main brush...much like adding texture to a brush. Say you have a 50-pixel mountain shape and your dual brush is like a 25-pixel mountain...only the pixels that correspond to both will be printed so you end up having to use a round brush large enough so that it doesn't clip off an bits. Then if you have large spacing the 2 brushes don't space properly as they are different size.

All in all dual-brushing doesn't work...I've beat myself about the head for many weeks trying to get it to work but it won't. The best you can get is setting the size randomness (which has a whole new set of pains to be endured -- if said tip is 50 pixels at one spot the spacing to the next tip is 100 pixels but when the next tip prints it's at 30 pixels and 200% is then 60 pixels so if the next tip is say 100 then the images produced tend to overlap). You can offset this a bit by setting the spacing to 300% or more but then it doesn't cluster properly.

So what I end up doing is a series of single clicks and if something overlaps I undo it. But it's still the same shape, which sucks, so it's best to learn GIMP or Paint Shop Pro for this kind of stuff. Or have tremendous patience.