Sources of light in a dark environment (nighttime, inside a cave, etc.) have specific ranges they illuminate. A torch, for example, provides Bright Light in a 20 foot radius and Dim Light for 20 feet beyond that. Past that range is Darkness, which counts as a Heavily Obscured area, equivalent to dense fog or opaque foliage, and which blocks vision entirely.
So if you have two people standing 100 feet away from each other, both with torches, each of them can see up to 40 feet away from their torch. The areas of lighting created by each of their torches do not overlap, so there is at minimum 20 feet of Darkness between them. Darkness creates a Heavily Obscured area, in which vision is completely blocked, again equivalent to dense fog or opaque foliage. With all that information, would you say that the two people can see each other's torches through the darkness?