First of all I want to say that this program is great and becomes even better every time!
So far Paintstorm is the only program I've seen so far other than Photoshop to have the "dual brush" feature. This feature in my mind is the main factor behind Photoshop's best "painterly" brushes.
In order to improve Paintstorm's engine to replicate what Photoshop is doing, I would suggest adding blend modes to the existing ones on the texture, and adding them to the double brush, for example, Photoshop has the dual brush with an overlay, color dodge etc which really creates some interesting effects and great brushes!
dual brush modes are: multiply, darken, overlay, color dodge, color burn, linear burn, hard mix, linear height
texture modes are: multiply, subtract, darken, overlay, color dodge, color burn, linear burn, hard mix, linear height, height
I think having all of these blend modes for double brush and texture in Paintstorm would really make the brush engine even more fantastic.