Thank you for the interesting solution… but there’s a bit of “cheating” going on there. First, you’ve moved my tween curves to the origin and aligned them in the same plane. Then you completely ignore all but the first curve’s planes. It’s clever, and I certainly learned something… can I ask two follow up questions?
How did you color the previews of the planes differently and scale them down?
What actually happens when you “reparameterize” the joined curves like you did? Can you divide them back in the end again and still get a line and an arc?
Now, I’d still like to learn the solution in my case as well, without moving any of my input curves… so far, I’ve got this, and I’m now trying to figure out how to rotate the highlighted planes to the projected points so I can then align the curves using those planes: