A curve that's a fixed distance from a Bezier curve is not in general something as simple as another Bezier curve.

You might try just adding a perpendicular to the tangent, or try the methods here:

http://www.cis.usouthal.edu/~hain/general/Publications/Bezier/BezierFlattening.pdf

You might try just adding a perpendicular to the tangent, or try the methods here:

http://www.cis.usouthal.ed