Whenever there is a double parsha, we generally read the haftorah of the 2nd one, but when achrei-kedoshim are together we read the haftorah for Acharei because we try to avoid reading the haftorah of Hatishpot whenever possible because it mentions the “Toavas Yerushalayim” (see Megilla 25b).
When Achrei and Kedoshim are separate, if one of them is a special haftorah such as Hagadol or Rosh Chodesh, we read the haftorah for Acharei on the other one.
The only time we would ever read Hatishpot is when the parshios are separate and neither has any special haftorah. Even then, there are some who have the minhag never to read who will just read the haftorah of Achrei twice in those years.
Which leads to another trivia question, other than reading the haftorah for Achrei twice in a row as mentioned above, when else do we read the same haftorah 2 weeks in a row?