It’s essentially as others say and as you’ve found out: you will get no protection ,wouldn’t if it was ba-ba on separate bookings either) and you won’t be able to add the new flight to your old booking (it likely would have been difficult or very expensive for two cash bookings anyway, two award bookings are the only easy thing here if there is space on both.

so you may prefer the vy flight for the same terminal and extra time but there will be no protection abc no through checkin for any option.