Haertel: Welcome To Mars and Galactic Cluster;
Loftus: The Star Dwellers and Mission To The Heart Stars;
Wald/Dirac: The Quincunx Of Time and Midsummer Century.
A curious feature of these diptychs is that each of them is also loosely linked to a single shorter work and that these three shorter works are all included in Anywhen which, like Galactic Cluster, is a collection, not a novel.
Welcome To Mars shares its background with "No Jokes On Mars";
the Loftus novels share their background with "A Dusk Of Idols";
The Quincunx Of Time refers to the background of "A Style In Treason".
In each case, there is a query as to whether the shorter work is fully consistent with the novel or novels in question. Welcome To Mars explains the Martian "canals" as natural phenomena whereas "No Jokes On Mars" refers to Canal Masons but both the novel and the short story feature the Martian dune-cats.
I will shortly reread "No Jokes On Mars" to check whether any further comparisons are appropriate.