How did Janus come to be emblematic of film?

[Mark Kohut]

My understanding, hearing one of the founders talk once, although memory is a fallible thing, is that they chose the name 'cause Janus Films was going to be an opening, a doorway, etc. into the "other' world of foreign  films; the other worlds they brought to the US....way back when....

I do not think they cared about all of the resonances and unanswered questions about two-sided godheads on coins but, for your coign of vantage:
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Janus was usually depicted with two heads looking in opposite directions. According to a legend, he had received from the God Saturn, in reward for the hospitality received, the gift to see both future and past.

In general, Janus was the patron of concrete and abstract beginnings, such the religion and the Gods themselves, of the world[6] and the human life[7], of new historical ages, economical enterprises. He was also the God of the home entrance (ianua), gates, bridges and covered and arcaded passages (iani).

He was frequently used to symbolize change and transitions such as the progression of past to future, of one condition to another, of one vision to another, the growing up of young people, and of one universe to another. He was also known as the figure representing time because he could see into the past with one face and into the future with the other. Hence, Janus was worshipped at the beginnings of the harvest and planting times, as well as marriages, births and other beginnings. He was representative of the middle ground between barbarity and civilization, rural country and urban cities, and youth and adulthood.

In Rome, Janus was worshipped in the Ianus geminus, a walled enclosure with gates at each end, situated in the Roman Forum which had been consecrated by Numa Pompilius. In the course of wars, the gates of the Janus were opened, and in its interior sacrifices -------------wikipedia

he's as rich as a Pynchon metaphor...I like that "one universe to another"