Funny thing!

 From:  FelixPQ (FELIX)

while working on this file I joined, I made a copy of the 2 solid on the right to preserve my originals and then union the copies, result on the left. Before I save these 3 solids for you to see, the object on the left was looking nicer or smoother then on the right but it had some artifacts. If you switch to the back view, you'll see the results seems perfect, the mesh angle is set at 5 and I tried it at 1 to see if it was only a display artifact.

The outer solid on the right is made using a sweep with 2 profiles and 2 rails and should look much better then this I think. This image here is what I was seeing before the save, as you can see there is something wrong inside the red circle, this is in front view, the back seems ok, nothing compared to the saved result. I made sure the profiles where symmetric both ways.

I made one of these before using unsymmetric profiles and had no problem, second image, but then I decided to make something like the red object and realized a symmetric profile would work much better to join 2 object like in test1.

Maybe this is a bug or something, I have no idea. If you need the curves and rails as well, I'll gladly send them to you.