Can you please share screenshots
Dual Brush is perhaps not the most intuitive of presets. Basically it uses the fisrt brush to constrain the extent of the dual brush. But you can adjust settings for primary and dual brush independantly.
So selecting a hard round brush as the primary preset, which gives a stroke the width of the upper line below, and then adding the leaf preset as the dual, and setting it so that it would normall spread beyond the primary brush, causes it to be cut off at the width of the primary brush. You are using the dual brush to place texture inside the primary brush preset.
I don't know if that helps, but as Ahmed says, a screen shot or two would definitely help us to help you.