The Gemini twins are private investigators.
They do not necessarily refer to themselves as the Gemini twins, that is just a name that got attached to them but it certainly is accurate. The older of the two is Meghan Petersen, usually going by Meg. The slightly younger is Beth. They are indeed identical twins, or at least were most of their childhood years. Since they are, when we first meet them, in their late 20's, each has acquired minor differences but for the most part, each still could be mistaken for the other, especially by strangers.
Our first encounter with them comes in an adventure with , a Spec-Ops legend who worked unofficially for a special adviser to the U.S. President. He was on a vacation hiking through Yellowstone when he encountered the pair as they were trying to not get killed by a gang of meth dealers they had disturbed. While Carter had saved their lives, considering the capabilities of the two sisters, who could say they would not have found their own salvation, considering their capabilities.
While the sisters lived and worked in the Boston area when we find them, they hail from Jackson, Wyoming where their father had been the head of the city's SWAT division, a bit of a legend himself. He had never thought of their entering the same line of work as he had but he had from their young days made it a point to teach them how to handle themselves in a fight should it become necessary and they had learned those lessons very well. Both were trained in Krav Maga and weaponry and neither were afraid of a tussle.
Neither were they ignorant of the dangers in one, though, as they learned when they were 22 years old and attending university. Several highly agitated young people had invaded the campus and went on a murderous rampage, determined to kill as many as they could before dying themselves. The SWAT team, led by their father, had ended the trouble but in the process he had lost his life and Beth has been shot in the head. Though Beth would recover fully physically from the injury, the damage had caused her to lose her memory of her life before the attack.
After the tragedy, the two moved to Boston where Meghan started a web design company and Beth joined her. It was on a vacation from that business that the trouble came that got them involved with Carter and from that meeting came the realization that life around Carter and his teammates, nicknamed the Warriors, was a whole lot more exciting in New York than their work in Boston.
So came the move to the Big Apple, going into the private investigation business as their new business, and making themselves as much of a part of the Warriors as the other had been. Thus while they were officially private eyes, that is just a small part of their activities.