Is it possible that Habsburgs and Hohenzollerns could agree a personal union in two of their heirs (marriage) and reunite their empires in a triple crown (Austria-Hungary-Prussia) at some moment in the 19th century?
Possible? Remotely, yes. Likely? ...The Habsburgs were a Catholic dynasty, while the Hohenzollerns were a Protestant dynasty.
Actually no - the Hungarians wanted to have their own Empire! Being part of the big German Empire would not severd the Hungarian aims.
But a "unification" of Germany and Austria maybe could have happed IF
1) Franz Ferdinand survives in June 1914 ==> no WWI
2) Franz Ferdinand becomes Emperor in 1916, his negotiations with Hungary and the Slavs in Austria in 1917 fail ==> Hungary declares independance, more and more Serb "terrorist attacks" and uprisings in Bohemia
3) At a "three Emperor conference" Germany, Austria and Russia agree on the seperation of Austria:
Russia gets Galicia, Serbia is allowed to purchase Bosnia and the remainig parts of Austria join the German Empire - Franz-Ferdinand accepts to become the subordinate of Wilhelm II because he realizes that he simply cant hold his Empire together - as a "gift" Austria recieves a part of Prussian territory, lets say Silesia.
I know not a probably outcome![]()
Possible? Remotely, yes. Likely? Not so much, actually. Their respective political and territorial interests were very opposite to each other, and bringing these differences together seems nigh impossible to me. And don't forget the most important reason why such a dynastic union propably wouldn't have worked out: The Habsburgs were a Catholic dynasty, while the Hohenzollerns were a Protestant dynasty.
Hohenzollerns had a Catholic branch in Sigmaringen I think.
And up to ~1800 it would have been a remotely possible idea to have a Union of the main branch of the Habsburgs and the catholic, Swabian branch of the Hohenzollern (the Brandenburg-Prussian ones belonged to the Franconian branch). perhaps even a stong Austrian presence in Southern Swabia centered on the Breisgau-Hechingen-Sigmaringen area.
But that has very little to do with the OP.
Anyway, the Wittelsbach were also a Catholic dinasty and accepted the Hohenzollerns as Kaisers, why not the Habsburgs?
There's a big difference between allegiance - which the Wittelsbachs had little choice about - and amalgamating the dynasties and realms. The ruling House has to be Catholic or Protestant.
It can't be Protestant, given the ongoing role of the Emperor in the Church (the Imperial veto of a proposed Pope was still in effect in the 1900s IIRC).
It can't be Catholic, because the Protestants of northern Germany won't accept it.
This is so easy. Have Maria Theresa marry Frederich the Great. OTL the marriage as considered but ultimately fell through, so just have it happen. Bam Union of the Habsburgs and Hohenzollerns.
Yeah that's the last chance. The 19th century is too late. But what a weird and unhappy marriage that would have been!
In a complex marriage contract, I imagine Frederick becomes a Catholic but religious freedom is guaranteed in both realms. Frederick is to become Holy Roman Emperor and retain his prerogatives in Prussia. Maria Theresa will inherit the Habsburg crowns.
I doubt they have children and I imagine the contract would address that. The heir is a Hohenzollern nephew as Emperor and king of Prussia but with limited powers in the Habsburg realms where the local aristocracies may have extorted concessions in exchange for the original marriage (?)