I looked up El Plomo on the map and it sits at 33 degrees of latitude. That is basically smack in the middle of the subtropical high, centered around 30 degrees of latitude. You can find more information by reading up on the Hadley cells of atmospheric circulation, if interested.
The main takeaway, though, is that air pressure tends to be slightly higher in these desert belts than near the equator or the mid-latitudes (maybe by a couple of percentage points).
I can't think of a reason for why two mountains that are in close proximity to one another would have markedly different air pressures at the same altitude, though... unless one of them was in the eye of a hurricane. Localized convection circuits, that vary based on small-scale weather and topography fluctuations could have an effect, but I wouldn't know how much and would guess that it isn't substantial.