It is the second option, you will get 2x Afterlife 2.

The thing that triggers an additional time is "that ability", based on the wording of Teysa Karlov. And "that ability" in this case is "Afterlife 2". So "Afterlife 2" triggers an additional time, meaning that on the stack you get both "Afterlife 2" and "Afterlife 2".

The first interpretation would require seeing Afterlife 2 as an ability that triggers twice, and then triggering 3 times instead. But nothing about Afterlife 2 is an ability that triggers twice. It is an ability that triggers once, and just happens to have an effect that involves the number 2.