Christian Solidarity Worldwide says Waqas Goraya and his cousin Asim Saeed, who live in the Netherlands, went missing in the Pakistani city of Lahore on 4 January.
It follows the disappearance of two other activists, professor Salman Haider Ahmed Raza Naseer, in the capital Islamabad days later.
The group, who regularly speak out against the country's blasphemy laws which are often used to target Christians, have been charged with blasphemy.
The group runs a blog which documents violations committed by security forces and religious extremists in Pakistan.
Protests have been taking place across the country against the abduction of activists.
Speaking to CSW, Pakistani human rights defender Tahira Abdullah said: "Pakistan must ratify the UN International Convention for the Protection of All Persons from Enforced Disappearance and the existing Commission in Pakistan must be made independent, autonomous and strong, in order to effectively handle the alleged cases of thousands of reported disappearances over the past 16 years.
"Freedom of expression is our inalienable and fundamental human right."
CSW's Chief Executive Mervyn Thomas said: "We urge the government of Pakistan to investigate both these and historic cases involving human rights activists in the country and work with human rights defenders to develop a national system for their protection."