Get the distance between the two.

Now make a horizontal line passing from the (x1,y1) & drop a perpendicular from (x2,y2) on it.

By this you get the two distances, (x1,y1) - (x2,y2)

and (x1,y1) - (xc,yc) where (xc,yc) is the coordinate of the point that you get by intersection of the perpendicular and the horizontal line.

as we know that cosine(x) = distance (x1,y1) - (xc,yc) / distance (x1,y1) - (x2,y2) in this case and thus you could easily get the angle between horizontal and the line.

Similiarly, perform same operation on another line as well . Add the two and you get the actual angle between the two lines.

Now you could find the half of that angle and draw a line at a given angle.

Bit confusing but i hope you get it.