Install on one drive normally.
After installation ends, do not shutdown/reboot.
- Copy all contents of installation
/boot folder to your SSD(main drive) existing $esp, or to a newly created one for exclusive use (fat32 of course).
- Find UUID of that $esp and create/add/edit the installation’s
/etc/fstab to have it mounted at
- Chroot into the installation, making sure the correct $esp is mounted at /boot.
- Create an entry in UEFI using efibootmgr, as per Archwiki, or
- Install grub as per Archwiki instructions
- If on Legacy/BIOS
Similar to the above, just the /boot partition is not $esp (fat32), just an ext4 partition.
And when chrooted, install grub on SSD.
Sorry, for the short versions of directions, IDK your Arch comfort level and you provided only few technical info.
If you still struggle, ask again.