It’s double the cuteness at the Franklin Park Zoo, where twin ring-tail lemurs were born last week. The babies, whose gender is not yet known, join eight other lemurs at the Franklin Park Zoo. All 10 can be seen in the Tropical Forest exhibit. Nebuchadrezzer, also known as Nebbie, gave birth to the twins Thursday. Officials said Nebbie is an experienced mother who has been doing everything she can to take care of her babies. “The twins are very active and are nursing well,” said John Linehan, president and chief executive of Zoo New England, which manages the zoo.