It seems that people who live at very high altitudes have demonstrably "thicker" blood, highly concentrated with red blood cells to overcome the low concentration of oxygen in the air. Moving between extremes of altitude apparently needs to be done gradually for the same reason.
Assuming a biology largely similar to humans:
Two (or more) humanoid species, in many ways biologically similar/compatible. One species lives almost exclusively in an isolated location at a very high altitude. One or more other species live in a variety of environments.
Is it plausible for species A to survive as such a height that members of species B were guaranteed not to survive in? Phrased differently, is it possible for a species to live in a habitat so extreme as to be radically socially isolating?