what happens if Amaterasu is cast on another Amaterasu?
To first answer this question; we must first look into "What can turn off Amaterasu"


Amaterasu lasts 7 days and 7 nights
Amaterasu can be turned off by the user's will
Amaterasu is turned off when the material projected onto it has been burned


With that being said, Amaterasu 2 will not "burn out" Amaterasu 1, they will simply burn for 7 days and 7 nights before they will both cease to exist.
Here you can read more about Amaterasu