Actually, instead of using brushes, you could create a base document on which you draw all your mountains one next to the other. On the layer below, you paint the inside of the mountains with white, and then you merge the two layers together. That way you won't see the hidden details of the first mountain when you put a second mountain over it top. Afterwards, when your entire mountain range is done, you can just "colour to alpha" the white out of the layer and what you're left with is a nice black outline without any overlap issues.