I will get right to it

The yellow spline is my original, also called the first spline

The blue spline is the 2nd i want to change so it maches the yellow one

What i am looking for is how to get so that the blue spline matches the yellow spline even if the spline locations for the blue spline is changed along the first spline

As you can see on the image the blue spline has its spline location 0 the same as the yellow spline location 0 & the blue spline location 1 is somewhere at a given distance along the yellow spline, we can say the half of the spline 1 location. And then i tried to match the blue spline tangents for spline point 0 & 1 but i can’t seem to get it right. It doesn’t match the original spline…

Importan to know, my blue spline has its spline points 0 & 1 always set along the yellow spline, so lets say my yellow spline has a length of 1500, then my blue spline has its spline point 0 set along the first spline at distance 200, then my blue spline again has its point 1 set along the yellow spline lets say its distance 800 (just an example)

Also look at the 2nd image for how i set the spline mesh (the mesh has more segment so it curves well)