This is what I did:
Add an entry to /boot/loader/entries (easy to copy the existing one) and simply change the name (called title) and the root=UUID= bit (using the PARTUUID from blkid /dev/sdXX for the partition).
options root=UUID=547465f2-f6f7-4677-82fd-f7b2e1ea3d8e rw quiet
So simple!!! The entry will appear in the boot list and it will boot to the PARTUUID.
NB. My 2 drives are both Antergos in the same state and I don’t know if the boot files are loaded from the 1st drive or after the Clone drive is accessed, but it works for me.