When you build a .abr tool preset tip you have an option for dual brushes... which is useful in building a watercolor brush. But if you switch to the mixer brush, this dual brush option grays out and you can't select it... Is this intentional?
> But if you switch to the mixer brush, this dual brush option grays out and you can't select it... Is this intentional? >
The Mixer brush is intended to paint with real paint characteristics. Instead of blending together two images to create the dab you paint with, you can use Texture to act more like a real brush painting over a textured surface, which will break up any smooth image, depending upon the settings. This is closer to how traditional brushes interact with the paint and substrate, being able to more or less mix the colors, and lay down the paint more or less evenly on a surface.
Try using Texture with patterns and varying the blend mode and depth, along with the wetness and load options in the Options bar. It can be quite exciting to see how varied these brushes can be.
It does mean it's not as easy to use any brush you've already designed, but you can download some brushes in CC 2018 from Kyle T (Brushes > Get More Brushes) and start working with those to see how they work — throw in a pattern in the Texture setting with any of them and fool around until you've got something worth saving—or just paint with his brushes as is.