The little pied tamarins were born on November 3rd to mom Twiggy and dad Socks. They also join a brother and sister in the family at the Rare Animal Conservation Center.
The Zoo says the twins have white fur on their head and cheeks that will fade by adulthood.
So far, their sex is unknown and they have not yet been named.
For now, both babies do a lot of hanging around, riding of everyone's backs. That routine, however, will soon change!
In about a month from now, the babies will start to act more independent and more adolescent. One of their favorite activities will be cutting in line, so to speak, to get their food.
"They'll start to steal food from the adults and, once they do that, they'll begin their training program with the keepers as well," said Anna Halko-Angemi of the Philadelphia Zoo.
The endangered pied tamarin is native to a tiny region of Brazil, along the Amazon River.
The fact the Zoo is adding to the species' captive population won big points with college students visiting on Thursday. They take a conservation biology course at Rowan University.
"They're bold, beautiful and in trouble, so I'm glad that the Zoo is helping them out and they're actually being successful in breeding them.
The best time of day to see the pair is early morning, as they retire to bed in the mid-afternoon.
Online: The Philadelphia Zoo