Suppose a boy was born Bein Hashemashos. Since this is a period of "maybe it's still day, maybe it's already night," seemingly the doubt would translate into "maybe he was born on day 1, maybe he was born on day 2."
Consider 13 years later. Since we're stringent regarding Biblical doubts, this boy would seemingly have to act on day 1 as if he is already a Bar Mitzvah, yet simultaneously would not be able to fulfill others' obligations on the presumption that he is not yet a Bar Mitzvah. By day 2 he's a Bar Mitzvah either way and the doubt is settled.
On Bein Hashemashos on the evening of this boy's Bar Mitzvah, between days 1 and 2, would he be able to fulfill others' obligations by a sfek sfeka?
By the same definition of Bein Hashemashos being "maybe it's still day, maybe it's already night," one can pose that "maybe day 1 is his Bar Mitzvah, and even if day 2 is his Bar Mitzvah, maybe it's already night and therefore day 2." The doubt is also reversible: "maybe it's already night and therefore day 2, and even if it's still day 1, maybe day 1 is his Bar Mitzvah anyway."
Or perhaps since both doubts ultimately stem from the same question of whether Bein Hashemashos is day or night, it would be considered one doubt and not two?