Are you aware of this kind of brushes?
I'm not looking for a brush that actually multiply itself, but that actually resembles watercolors.
Not available for Photoshop, as far as I am aware. It can of course be faked with water colour brush sets, but if you want TRUE physically simulated water colour painting, then you would have to look at either Expresii or Rebelle. Expresii even supports tilting your tablet to control the flow of the water(!).
it is just what I was looking for.