Hi bigseb - the red curve would need to be modified in order to be tangent to the upper green curve.

Instead of modifying the existing red curve, it's probably easiest to use the Blend command to construct a curve that has the tangency you need.

To do that, trim the upper green curve at the point where you want the new curve to start, you can use the "Add trim points" button in the Trim command to specify a point on the curve for where to cut it.

So after that you would have an arrangement like so:



Select these 2 curves:



Then run the Construct > Blend command to build a blend segment that is tangent between both of them:



- Michael