White line between theoretically joined objects.

As in the video below.
https://youtu.be/S4rhj6IHWHA

I have created two objects that are kind of rectangular.

When I try to join the two objects, there is always a faint white line between them. Even though theoretically they are joined.

Before you ask, I have already selected both objects and enabled "Object Hiting", but this annoying white line between the objects is still there.

Any solutions please?