Series With Only Two Installments

Surprisingly, several single works by Poul Anderson each turn out to have an unexpected prequel or sequel hidden away in some other collection. If a series comprises at least two items, then there are several short series not usually recognized as such.

Prequels
"Pride" (see also here and here) is a prequel to Tau Zero.
"The Innocent Arrival" is a prequel to "The Moonrakers."

Sequels
"The Ways of Love" is a sequel to The Enemy Stars.
"Quest" is a sequel to The High Crusade.
"War-Maid of Mars" is a sequel to "Duel on Syrtis."

The Star Fox and Fire Time, which I collectively call Star Time, are a two-volume series but not a two-installment series because The Star Fox is itself a collection.

These reflections are prompted by my continued amazement that "War-Maid..." is a sequel to "Duel..." Both stories refer to the Hemispheric Wars and to the Martian environment whispering messages. When the viewpoint character of "War-Maid...," meeting the Martian resistance leader, thinks, "So this was Kreega, the great enemy of Earth...," that name should mean something not only to the Earthmen whose rule Kreega resists but also to the reader if s/he has read "Duel..." sufficiently recently.

The title character of "War-Maid..." is a human being who calls herself a Martian because she has grown up on Mars where there has been cultural integration between natives and colonists. Thus, she resembles Tabitha Falkayn who identifies as Ythrian.