"Common Time": a first faster than light interstellar crossing with the Haertel overdrive;
"Nor Iron Bars": a second FTL crossing with the Arpe drive;
exploration of the microcosm with the Arpe drive;
discovery of the microcosmic basis of telepathy;
"This Earth Of Hours": FTL on the Standing Wave;
early contact and conflict with a telepathic interstellar empire.
Since "Common Time" features the scientist Adolph Haertel and since Blish's juvenile novel Welcome To Mars features the teenage Haertel, the latter volume can be seen as a prequel to the Galactic Cluster trilogy but there the linear sequence ends, with Blish's "Haertel Scholium" volumes presenting several alternative or divergent futures rather than a single series.
Because, in the Haertel Scholium, it is Thor Wald that invents the Dirac communicator and because Wald, while appearing in one volume, is referred to by name in another, he might count as the unifying character of his diptych as Adolph Haertel and Jack Loftus are of theirs.
I have yet to finish rereading and posting about Welcome To Mars but meanwhile wanted to commend the short trilogy to which it can be seen as a prequel.