it is to setup one big LUKS partition in each disk, and put a LVM2 Physical Volume inside each partition. At boot, the two LUKS partition are opened, their contents are assembled into a Volume Group, and I can have everything I want inside.
... sans entrer le pw plusieurs fois ...

intéressant, btrfs au lieu de lvm
https://www.redpill-linpro.com/techblog/2016/08/12/btrfs-and-encryption.html

et toujours la ref de Arch https://wiki.archlinux.org/index.php/Dm-crypt/Encrypting_an_entire_system#Encrypted_boot_partition_.28GRUB.29