As attached figure the line is tangent attached once at one inflection.

The task is how to attach line tangent to two inflection points. In mathematical Line should be tangent in two points to spline and should have only two common points with curve

Ok was just able to try it. I see what you mean. I thought it would allow another relationship for each segment between node on the spline but it doesn't. Annoying. Jeremy's solution was easy and worked well for me.

use 2 co-linear line segments