Sharing Planets

Several times, Poul Anderson shows two rational species inhabiting a single planet. In his Technic History:

Starkad has both land-dwellers and sea-dwellers;
Talwin has both hibernators and estivators;
human beings colonize the plains of the planet Altai whereas natives with below zero body temperatures inhabit frozen mountain tops;
human beings and Ythrians jointly colonize Avalon;
some Merseians emigrate to the human colony world, Dennitza.

In Anderson's Three Worlds To Conquer, which I must reread, a Jovian defence force suddenly realizes that the crews of an approaching invasion fleet are of a different species that must have evolved on another continent of their massive planet - as if we could meet extraterrestrials by crossing the Atlantic.

In Anderson's Fire Time, human beings and Naqsans come into conflict on a planet that both have colonized whereas human colonists of Ishtar admire and coexist harmoniously with native Ishtarians. On Avalon, Dennitza and Ishtar, Anderson shows us two species growing up together while mingling or merging their cultures. As a very small example, the human word "Okay" has entered an Ishtarian dialect. An Ishtarian male and his wife discuss their shorter-lived human friend, Jill. He hopes that Jill will have children because, living for centuries, he befriends several generations of a single human family.