looked challenging at first, but its not that bad.
the midpoint between your 2 points is where to draw the billboard.
its coordinates are just the average of the coordinates of the two points p1 and p2:
then you need a billboard quad.
if is just something simple like a 10x10 quad, no problem.
if the distance from p1 to p2 defines the width of the billboard, its a bit more work.
so the next question is: what size quad do you want to draw?
once you have a quad mesh and a point to draw at, aiming it at the camera is trivial, billboard xr = cam xr + 180, billboard yr = - cam yr, as i recall.
so, what size quad do you need?
its not too hard to create / scale a quad based on the distance between p1 and p2 if that's what you need.