An easy solution that works really well (i use it myself) is to draw two lines on top of each other.
The trick is to set the opacity on both lines at ½ the final opacity, fx 0,5 if you want a solid line, and making one line a bit bigger that the other.
draw_line(from_pos, to_pos, Color8(255, 255, 255, 255/2), 4)
draw_line(from_pos, to_pos, Color8(255, 255, 255, 255/2), 3)
Before and after example from my game (Code above is just an example, not exactly what i use):
I am not 100% sure this technique can work for your exact needs but i thought i would throw it out there anyway ;)