double curved lines

hi does anyone know how to do that so they run parallel? I tried making a custom brush with two brushes but it doesn't really work when it curves round itself. I want to make a smooth outline parallel to the shape I have drawn and be able to change the thickness of it. Any ideas?

thanks Herbal

Make a mask from the shape. Expand the mask. Stroke the mask with desired line width.

thanks very much LonK I'll see if I can do that