Best way to do it would be to order a "Additonal ONT" and relocate the second ONT into the second dwelling.
This would be a nightmare for any service moves though, most agents who deal with only resi won't be used to dealing with multiple onts and ensuring they touch the port with correct ONT/asid.
I've seen many landlords abuse the crap out of this though, some even going as far as to run CAT5 from their ONT to the second dwelling and going "you can only order secondary services" - which heavily limits your providers and available services.
is there an alternative without subdividing? we've also been told that the 'additional ONT' would have to be in the same building as the first one.