I tried myself by splitting the mesh and join it.
So I should have 2 seams.
Yes, I have and UnWrap goes well.
Then I tried with 1 seam only.
Also works well.
To select 1 seam only is cumbersome.
The chain option while selecting the seam is useless.
In the end I selected both seams by window, and then Ctrl-windowed the unwanted one.
I’m sure you know a smarter method?