On Thursday, the Internal Security Agency (ISA) said it had arrested the assistant director at the same private language school in Tripoli where the first man was arrested, identifying him by the initials "SBO."
The ISA accused him of operating "in the company of his wife as a missionary on behalf of the organization 'Assemblies of God' in order to seduce the sons of our Muslim people."
Assemblies of God is a Christian missionary organization based in southern US state of Arkansas.
Libyan security forces said Thursday it had also arrested two Libyans, including a 22-year-old woman who had allegedly converted to Christianity when she was 15.
In a video, she describes how she in turn became a "missionary" and tried to persuade other Libyans to switch faiths.
It is the latest of a handful of video "confessions" of "apostasy" in Libya.
While the security forces have not named the man arrested on Wednesday, Libyan media identified him as Jeff Wilson, the founder of the consulting firm Libya Business.
Islam is considered the state religion in Libya, and while Christians have the freedom to worship there, the majority are foreigners living in the country.
