Is it advisable to combine two dataset?
Adding to what @Super_John said, if adding continents as a Feature, then you can also probably have at-least 2 more features as well,


The Latitude
The Longitude


Also add another temporary column to indicate the Source (like $1$ to $1st$ df, $2$ to $2nd$ df etc), So that we can add Colors to the k-means

So now we can have a k-means Cluster to see whether values are overlapping or not...
(Trying to see it in an Unsupervised Way)

(The analogy is equivalent to the  fact that that you can cluster time(24  hours in a day) in a cyclic fashion , like plotting  $sin(x)$, $cos(X)$ and then trying to cluster them)

Have a look at this answer,
Features Selection, Extraction