When Luhmann refers to ego and alter, as in discussions of double contingency, it is easy to assume that he means psychic systems as in interaction systems of two people. But according to Luhmann,
The social dimension concerns what one at any time accepts as like oneself, as “alter ego” [. . .]
To begin with, it is important to avoid combining the social and fact dimensions. This was the cardinal mistake of humanism. [. . .]
The distinction between factual and social dimensions should not be misunderstood as the distinction between nature and humankind. [. . .] The social dimension [. . .] emerges from the fact that alongside the ego-perspective one or many alter-perspectives come into consideration. A social reference can then be required of every meaning. This means that one can ask of every meaning whether another experiences it exactly the way I do. [. . .]
Accordingly, the concepts of ego and alter (alter ego) do not stand for roles, persons, or systems, but for special horizons that collect and bind together meaningful references. Thus the social dimension is also constituted by a two-fold horizon; it is relevant to the extent that in experience and action it becomes apparent that the interpretive perspectives a system relates to itself are not shared by others. Here as well, the horizonality of ego and alter means that further exploration will have no end. Because a twofold horizon is constitutive of the independence of a meaning dimension, what is social cannot be traced back to the conscious performances or a monadic subject. This has been the downfall of all attempts to establish a theory of the subjective constitution of “intersubjectivity.”Social Systems, 80-81
So ego and alter constitute a twofold horizon, and horizons are uncrossable; they keep receding. Any person’s view of the world is contingent; it does not have to be what it is–i.e., it isn’t necessary. There are other possible interpretations.