If strong fences make good neighbors, Chile and Argentina should be blood brothers.
The Andes form one of the most impregnable natural boundaries between any two nations on earth, effectively isolating the two neighboring countries. Any mountain passes between the two are snowed-in for much of the year, with very few passes even theoretically open year-round (and even those are regularly closed by inclement weather and/or avalanches).
Beyond making war nearly impossible, overland trade, cultural links and economic integration all are incredibly limited. While sharing a common language and colonial history, the Chileans and Argentineans have almost nothing to do with one another.