Hi,
In captivity, same sex cockatiel will often bond like a male/female, and mating isn’t uncommon. It’s harmless unless one bird is being aggressive or obsessive. If the mating is over and over and the other bird is being chased, then you can separate them. It is also possible that you have the gender wrong on one of the birds and actually have a female. Not all cockatiels can be sexed by appearance. With some mutations there is no difference between the male and female markings, and sometimes the markings can be misleading. If the mating continues, you may end up with eggs and there will be your answer. If you want to know for sure and don’t want to wait to see if one bird lays eggs, you can have a DNA test run to determine gender.
Thank you for asking Lafeber,
Brenda