I think she's referring superficially to a generic divide, not specifically the continental divide. A divide is geographically any ridge or raised area that is separate from the rest of the landscape; Cather may have been using this to talk about both the location and psychology of her characters. They came from a European life to one completely different in Nebraska and had to adapt to that new way of life. Possibly she was using the title as a reference for both. Possible I am also overcomplicating. :-)

Possibly she was using the title as a reference for both.

Is it common for authors to use a proper noun, i.e. "the Divide", to refer to two or more separate ideas together? If so, does this technique or device have a name or other examples I can look at?

Thanks for the wisdom, DeepFriedTwinkies. Honestly, I don't think either one of us are being complicated enough. ;-)

Watershed divides exist in addition to the continental divide. I am along the Rock River, which flows south into the Mississippi and the Gulf, but a little bit east of here the streams flow east to Lake Michigan and out the St. Lawrence Seaway. Getting across this divide was a challenge for early explorers using these waterways, so they were quite aware of where they were.

There is a watershed divide between the Missouri river and the Arkansas river, which runs through south Nebraska. It is this which gave its name to the "Nebraska Divide" and Divide, Nebraska. In this map the "Republican", "Little Blue" and "Big Blue" watersheds flow south, not into the Platte.

Is it common for authors to use a proper noun, i.e. "the Divide", to refer to two or more separate ideas together? If so, does this technique or device have a name or other examples I can look at?

The term you're looking for is "metaphor" ... honestly, only pedestrian authors have one level of metaphor in mind when they're writing. Complex, intellectual authors like Nabokov or Kobo Abe have intricate metaphorical structures that can mystify, confuse, illuminate, surprise and startle you years after first reading them. Granted, Cather is not that complex, but I think DFT is basically correct in his assessment, and I can think of at least three major ways in which the "divide" is evoked by the situation.