Rupert And Pym

Poul Anderson's A Midsummer Tempest has a Cavalier hero whereas John Sanders' Nicholas Pym series has a Roundhead hero so let's have a trilogy:

Vol I, Cavalier hero defeats his Roundhead enemies;
II, Roundhead hero defeats his Cavalier enemies;
III, the two heroes go head to head -

or even a Tetralogy:

IV, what happens to everyone after the Restoration -

or, indeed, the story continues:

V, there are royalists and republicans in later history down to today.

Pursuing the idea further: Vols I and II written by different authors and III a collaboration.