PREPARATION:
0.) Comment out all /boot mounts in /etc/fstab and umount /boot if mounted as a separate mountpoint.
    You want to use *the /boot on your / mount*.

1.) Prepare each target partition (partitions/part below) as an ESP
    (https://wiki.archlinux.org/index.php/EFI_system_partition#Format_the_partition).

2.) Install GRUB2 to *each ESP*. See sample.config.xml for context for the below examples.

  grub-install \
      --boot-directory=/mnt/boot1 \
      --bootloader-id=Arch \
      --efi-directory=/mnt/boot1/ \
      --target=x86_64-efi \
      --no-nvram  \
      --recheck

  grub-install \
      --boot-directory=/mnt/boot2 \
      --bootloader-id="Arch" \
      --efi-directory=/mnt/boot2/ \
      --target=x86_64-efi \
      --no-nvram  \
      --recheck

# These are not strictly necessary, as the same path is used in efibootmgr for the primary and the fallback.
#   grub-install \
#       --boot-directory=/mnt/boot1 \
#       --bootloader-id="Arch (Fallback)" \
#       --efi-directory=/mnt/boot1/ \
#       --target=x86_64-efi \
#       --no-nvram  \
#       --recheck
# 
#   grub-install \
#       --boot-directory=/mnt/boot2 \
#       --bootloader-id="Arch (Fallback)" \
#       --efi-directory=/mnt/boot2/ \
#       --target=x86_64-efi \
#       --no-nvram  \
#       --recheck

3.) Prepare the ESPs. See sample.config.xml for context for the below examples.

  efibootmgr \
      --create \
      --disk /dev/sdd \
      --part 1 \
      --loader /EFI/Arch/grubx64.efi \
      --label "Arch (Fallback)"

  efibootmgr \
      --create \
      --disk /dev/sdb \
      --part 1 \
      --loader /EFI/Arch/grubx64.efi \
      --label "Arch"