Three Integrated Series

Poul Anderson's History of Technic Civilisation began when Anderson, on impulse, made Dominic Flandry, the hero of one series, refer to Nicholas van Rijn, the hero of another series now to be regarded as set not in an unrelated timeline but in an earlier period of the same timeline. However, the History is more than these two series strung together. It incorporates other works, including a third series. Two stories describe early human contact with winged Ythrians. Two further stories plus one novel are set on the joint human-Ythrian colony planet of Avalon. Thus, this "Ythrian" series comprises:

"Wings of Victory"
"The Problem of Pain"
"Wingless"
"Rescue on Avalon"
The People of the Wind

Then:

the entire period of the Polesotechnic League, including "The Problem of Pain" and the life of van Rijn, occurs between "Wings of Victory" and "Wingless";
the entire period of the later Terran Empire, including the life of Flandry, occurs after The People of the Wind;
one story of early interplanetary exploration precedes "Wings of Victory";
two stories covering the founding and early days of the Terran Empire are set between "Rescue on Avalon" and The People of the Wind;
four works are set after the Empire.

Thus:

"The Saturn Game"
"Wings of Victory"
the Polesotechnic League, including "The Problem of Pain"
"Wingless"
"Rescue on Avalon"
two stories of the early Empire 
The People of the Wind
 the later Empire
four works set post-Empire

Thus, the Ythrian series provides a framework around which the rest of the History is constructed. Van Rijn and contemporaries of Flandry meet Ythrians. Further, a collection, The Earth Book of Stormgate, begins with the first two Ythrian stories, ends with the third and fourth Ythrian stories and between them includes eight works set in the Polesotechnic League period. A fictitious Ythrian editor comments on the stories even revealing that some were (fictitiously) written on Avalon. Thus, an Avalonian Ythrian perspective is allowed to inform about half the narrative of the Polesotechnic League, including "Lodestar" about the planet Mirkheim, which is a turning point for the League, and "Day of Burning," which describes a turning point for the planet Merseia, the later rival of the Empire. Ythrians, although less prominent than either van Rijn or Flandry, are a major component of the fabric of the History.

We learn that:

van Rijn's protege Falkayn founded the Avalonian colony;
another protege, Dalmady, moved to Avalon where his daughter wrote one Earth Book story about Dalmady's encounter with van Rijn and another about Falkayn's grandson on Avalon;
the Ythrian editor and his human collaborator wrote the accounts of Mirkheim and Merseia from material provided by van Rijn and Falkayn. 

Anderson's skilful integration of disparate elements into a single fictitious history is more effective than Asimov's clumsy bringing together of Robots and Foundation:

Anderson
Ythrians
League
Terran Empire

Asimov
Robots
Galactic Empire
Foundation