ok. I've just solved it.
I think that I must not delete surfaces where on both solid.
And I must select just curves.

Thank you.

PS: What a speedy forum :)

Hi Hunter, sounds like you have figured it out, but the key thing there is that you need to select the "edge" of a surface, and not just a free-standing independent "curve" object which can sometimes be stacked in the same location.

Construction curves can tend to get in the way of picking edges for other operations too like Fillet so it is not a bad idea to delete or hide them after you use them to get them out of the way.

- Michael