Hey, I know this is well and truly necroing this thread and sorry if this is common knowledge but I've only just seen Thomas' question regarding the overlapping of brushes and I've dragged a method out of the vaults of my memory that no-one seems to have mentioned here...

I cannot remember who originally came up wirth the tip but you can use the Clone Stamp tool (talking PS here) to act as a multi-layer brush.

Make yourself a palette of brushes out of the way, with space between each one so you can select them one at a time. They need to be drawn as you want them to be pasted. So a transparent background, and the brush opaque. Make sure you have the Clone Stamp set to "Current and Below" for the layers and have "Aligned Sample" unchecked. Also make sure opacity and hardness etc. are ramped up to 100%.
Then simply alt+click on the brush you want and then pop some where it needs to go. To change brush simply alt+click on a different one on your palette. Because the brush is opaque there will be no previous brushes showing through.

I'm pretty sure I used this technique myself on one of my earlier maps.

Anyway, hope this helps anyone who still has this problem.