Hi Pilou,
the first image shows how the curves where constructed and when joined the resulting sweep with all the seams.
The second image is the joined curve that as the rebuild command applied and only one seam which position you cannot control.
the third shows the rebuilt curve split where you want seams to be then joined and when swept give 2 seams.
The fourth shows the third curve with 1 seam point removed.
Hope this helps
Barry