You should have the same problem: white should be completely transparent.

I just tried the technique I suggested, and it doesn't work unless the two brushes are exactly the same size. I'm not sure where I went wrong defining them, but they were a couple of pixels off, so the first mountain was perfect, and each one thereafter was just a little bit more wrong. Probably more work than it's really worth.

What's frustrating is that this is one of those things that Gimp does fairly easily. I don't know why Adobe can't manage to implement features that an open source product contains. Or maybe this is just their way of trying to get people to also buy Illustrator, in which it's as easy as dragging your shape to the swatches panel.