I think I'll take a dive into this, starting with the alternate version of one of my favorite series of novels:
The Harrington/Theisman Series, By David Weber
The Harrington/Theisman series is a well-known, if slightly controversial, military science fiction series by American author and military historian David Weber. The series initially revolves around the conflict between the Star Kingdom of Manticore and the United Republic of Haven, both parallels to 18th-century Britain and France, respectively - and the relationship between the eponymous protagonists, Helen Harrington and Thomas Theisman. Both societies are aristocracies, explicit in Manticore's case, de facto in Haven's case. Both Harrington and Theisman are brilliant naval officers from common backgrounds. A major theme is the two of them managing to work together despite their superiors.
The first book, On Basilisk Station, sees the two of them assigned to a joint Manticoran-Havenite operation overseeing the Medusan colony. The Manticorans want to exploit the Medusans, while the Havenites want to "protect" them by denying them all access to modern technology. Both sides are also looking to start a war, using Medusa as a flashpoint, but Harrington and Theisman manage to avert it, and improve the lot of the Medusans in the process, although they never actually meet. The book ends with Medusa joining the Star Kingdom under the rule of an enlightened Medusan monarch who swears fealty to King Roger. Harrington gets the credit and is properly honored, while Theisman manages to deflect the blame onto his superior officer.
The second book, Duty and Honor, sees Haven and Manticore sparring over the strategic planets of Grayson and Meggido. Grayson is essentially a giant commune with a theocratic overlay, while Meggido is an oppressive and horrific theocracy. Theisman is sent to "join" the Meggidoan Navy while Harrington brings aid to the Graysons. Both are initially disgusted by the societies they find, although Harrington slowly warms to the Graysons. During a Haven-backed Meggidoan attack on Grayson, Honor and Theisman face off in battle for the first time. Theisman wins and captures Harrington, but he protects her and her female officers from the Meggidoans. When the Meggidoans try to seize Theisman's ship, Theisman and Harrington escape back to her ship, which Harrington uses to blow the Meggidoans apart. Harrington is made Hero of Grayson, complete with , Meggido is conquered by Haven, and Theisman goes home in disgrace. He is demoted to Lieutenant.
The third book, A Short Victorious War, begins the main story arc of the Manticoran-Havenite War. It introduces Harrington's academy roommate, best friend, and former lover Michelle Henke, who is herself part of the Manticoran royal family, but is a good sort nonetheless. It also brings back Harrington's nemesis, Lord Pavel Young, who tried to rape her back at the Academy and was her superior in On Basilisk Station. Young starts the Manticoran-Havenite War by being a cocksure dumbass, while Theisman gets involved in a plot to start a revolution. Harrington manages to save most of her ships by disobeying an order from Young, which gets her effectively cashiered (she was up for mutiny, but half the admirals on the court were actually honest and saw that if anything, it should be Young who gets the noose).
The series continues with Harrington going to Grayson for a while, while Theisman watches as the Havenite Revolution turns into a horror show, with entire families being executed, commissioners on ships questioning the captain's orders, etc. It's a cross between Stalin's Russia and Robespierre's France. Theisman and Harrington meet again, this time with Harrington taking Theisman prisoner, and the romantic chemistry between them becomes clear. Harrington returns to Manticoran service eventually, only to discover a conspiracy within the top reaches of the Manticoran government. She eventually traces it to the slavers and genetic engineers of the planet Mesa, and engineers a coup that puts Michelle Henke on the Manticoran Throne and severely cuts back the power of the aristocracy. Theisman, meanwhile, aids a second Havenite revolution that establishes a government clearly based on the UASR. When the massive Solarian League attacks Manticore in the novel At All Costs, Harrington and Theisman bring the Grayson and Havenite fleets to its aid, defeating the Solarians and establishing an alliance between the reformed Manticore and Haven. Needless to say, the book ends with the two of them making out in front of the media. The final books see Manticore and Haven join forces against the Solarians and their Mesan masters, with Harrington and Theisman working side-by-side (and coming up with a constant stream of excuses to see each other).