Two curves which, at any point, have a common principal normal vector are called Bertrand curves. The product of the torsions of Bertrand curves is a constant.
Bertrand Curves
Explore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Bertrand Curves." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/BertrandCurves.html