Sorry, but it just won't work in Gimp given the way brushes work. The closest I've come with a double track brush is to make it bow-tie shaped with faded edges to allow for the angles without overlapping. You don;t need to make a GIH, just a single brush and enable brush dynamics, pickling "Track Direction", which will do the rotating for you. But it will never form clean cart tracks/lines.

If you are comfortable working with paths, it is better to to this making three strokes along the same path - one wide soft stroke in the lighter colour to make the outline, then one harder dark stroke to make the main track, then a smaller hard stroke in the light colour again to "clear" the space between the tracks. Here is an example done that way:
tracks.jpg

-Rob A>