Zweihänder setting

The obvious way:
A GRAND TOURNAMENT

The cities are empire-like, the rural areas are Brettonia-like. The nobility of the sword protects the borders with a series of fortresses, each lord maintaining a retinue of experienced warriors to call in times of danger. The cities, on the other hand, are increasingly dominated by wealthy merchants, who have begun investing in professional armies of mercenaries to protect their caravans in foreign lands and have been financing research into advanced gun powder weapons with which to equip the militia. The high king and his council struggle to keep the balance between the rural and urban power centers, with the possibility of a civil war erupting if both sides aren't placated.

None, your are the DM. Tell them they are contracted to travel to the new world and help to sack the mystical temples, so they can both use character from an empire like natiopn and a bretonnia like nation.

This is best answer. Similiar answer.

Have the nation be played during a time of revolution. Old timey nobles want war kept their domain. Rich merchants / young noble houses / innovative old noble houses want to progress.

The kingdoms royalty is split. Wanting obviously to be powerful but fearing if it welcomes too much change the conservative nobles will rebel.

Tldr; a european version of the last samurai

Or just do it like literally your picture with 2 empires "coexisting".

How do I Belgium when worldbuilding?

wat zweihander?

Bigass sword from around 1500

So Empire?

This. You even saw the beginnings of this happening in Brettonia with the rise of "clubs" like the Brotherhood of the Lighthouse that were really merchant guilds in all but name

drivethrurpg.com/product/210516/ZWEIHNDER-Grim--Perilous-RPG-FULL-VERSION

Is it good?

Border Princes is a warhammer fantasy, probably stolen from somewhere else like most of their stuff. It mostly refers to a lawless, rough, poorly settled area on the edges of the kingdom populated with bandits, orcs, desperate people etc. that kings/emperors/whatever send ambitious but landless nobles to so they can go be ambitious somewhere far away from court. If they do well they take care of threats and you can start taxing their now claimed land, if they die it gets them out of your hair. Leads to a wild west, frontiersman adventuring sort of environment that can easily accommodate bretonians and empire folk in the same place.

Historically there's stuff like that all over europe, basically where ever the borders between two places are a bit loose and there are too many young warriors with not enough shit to do. Read some history books and steal their ideas and/or read the whfrp Border Princes book. Its good for world building. Osprey isn't the most rigorous historical stuff, but they're short and give you a decent idea and have cool pictures.

Just set it in AoS dude. The setting is much more suited for roleplaying, and it doesn't have either so you wont have any arguments.

sooo....wfrp 1e heartbreaker?

Look up the Duchy of Burgundy. The best parts of France (Bretonnia) and the Holy Roman Empire (The Empire) combined into one powerhouse of a medieval state. They had a proud knightly tradition combined with one of the first attempts at a real standing army, complete with artillery train and what have you.

myarmoury.com/feature_armies_burg.html