The Last Viking (New York, 1980) refers to the title character of Mother Of Kings. Harald Hardrada:
"...remembered Gunnhild the witch, wife of Eirik Blood-ax." (pp. 86-87)
Harald compares Gunnhild to the Eastern Roman Empress, Zoe. We see the last days of the Western Empire in Poul and Karen Anderson's The King Of Ys and of the Eastern Empire in Rogue Sword. The title character of The King Of Ys refers to Marius who defeats the barbarians in The Golden Slave.
The Golden Slave is one of three novels set BC,
The King Of Ys is a Tetralogy,
Mother Of Kings is one of five interconnected fantasy novels set in the Viking period,
The Last Viking is a Trilogy and
Rogue Sword is one of three novels set in the fourteenth century,
these eighteen volumes present a panorama of the past comparable to the futuristic panorama presented in Anderson's History of Technic Civilisation.
There are differences. The Technic History, comprising more than one sub-series and many individual works, remains a single diverse science fiction series whereas the past "History" is not a single series and its constituent works are not even of a single genre. In fact, if, as I believe, Mother Of Kings, which I have yet to read, is a fantasy, then its Gunnhild cannot be exactly identical with the witch queen of that name referred to in a work of historical fiction.
Nevertheless, I think that a Complete Works of Poul Anderson could usefully present these works in their historical order. The reader would then see history develop and would appreciate the cross-references as they appear. Of course, the other way to present a Complete Works is to follow the author's development by arranging the works in order of publication but this approach, if taken to its logical conclusion, would split up the works that are meant to be read as series so that, for example, Ensign Flandry would be read after some later phases of Flandry's career.
I apologise to any regular readers of this blog who will certainly tire of this aspect of Anderson's works before I tire of writing about it.