Sarah and Simon are agents of DAAAD.
Agents-in-Training, to be more exact although as we initially encounter them in the first recorded adventure, they hope they are about to get their first mission off-island showing that training is nearly over. Sarah complains before they are told about the upcoming assignment, "does DAAAD trust us to actually go out on a mission? No. No, they do not. They treat us like children. What's the use of having amazing skills if we can't get out there, right? All we do is train. We're like circus animals, performing inside a show ring".
First, a tad about DAAAD. It is a (?private?) intelligence organization performing assignments on demand all over the world. Its base of operation is an island - apparently a decent sized one complete with a moderately active volcano. Its full name is the Disinformation Assessment and Administration Department, a title that does not really tell us all that much about the group.
Sarah and Simon tell us they 'belong' to DAAAD and that they never knew who their parents were - they wonder if they even had legitimate parents. The volcanic island mentioned has been their home since the day they were born. That birth was 17 years before the day we meet them, to the day. It is their birthday, or at least that is what they have been told and they choose to believe it. "We don't have any birth certificates in any database anywhere, but there's no reason to doubt what we've been told about our birth. I mean, we're obviously twins; the same thick, dark hair, the same brown eyes, the same long, straight noses and wide chins, and the same olive-toned skin.
Sarah is the older of the twins by nine minutes and she makes sure everyone who matters knows it. According to Simon, she lives her life like she's the star of an action movie. Sarah, who never fails at anything. Sarah, who would throw herself on top of a grenade for me, but won't share her sandwich. We've always been together and always will be. I'd follow her anywhere".
Simon is the less adventurous one - although not by a whole lot. He thinks of himself more of a poet and a gadget-man, coming up with improvements and enhancements to the really cool stuff that DAAAD comes up with.
They have a YouTube channel - rather unusual for people wanting to be 'secret agents' - but to be fair, they keep their identities hidden. It is named CrazyStutz and they have had it since they were 12. It not only gives them something to blow off their energy but they also find it a great way to use their "well-honed spy skills". These two have a lot of excess energy to dispel - all preparing and eagerly awaiting the day of their first mission.