How do I draw two parallel poly lines based on only two points (similar to google maps traffic)

Question

I have a Polyline that is drawn from point A to point B, I'm trying to figure out how to draw two lines of a constant width that appear something like this image.

I only have two points, rather than four, I thought maybe there was a way to create four points, based on the two and then draw the corresponding lines, but I cant seem to get my head around it.

Any suggestions