Hi Pilou,

re:
> When you say "Join to glue them together into 2 closed curves" it's the 2 family's curves Joined
> in the same time or Join family curves by family curves one by one?

I mean with these 16 edge curves selected:



Run the Edit > Join command one time, that will generate a result of 2 closed curves each made of 8 segments. Then you can do one loft between those 2 closed curves rather than doing 8 lofts.

- Michael