AFAIK South and North America belong to two independent tectonic plates that happened to merge a few millions of years ago; North America was part of Laurasia,

AFAIK South and North America belong to two independent tectonic plates that happened to merge a few millions of years ago; North America was part of Laurasia, while South America was part of Gondwana and it was originally attached with Africa. So I guess you can indeed argue that North and South America are two different continents...