...have an Austrian Empire split relatively equally in terms of its power potential between territories in Italy and OTL 1900 Austria-Hungary.
I think it's possible by 1800, but during the 19th century became too strong a social force in western Europe for Italian speakers to accept a German government or German speakers to accept an Italian one.
If you have an Italian unification movement arise in the south of Italy in the mid 1700s with stronger agrarian policies and a sterner Christian rule philosophy (maybe they twist Rousseau's idea of the noble savage an apply it to 'moral' farmers vs 'lacivious cities' like you find up north) then you have the northern cosmopolitan Italian cities repel in horror.
You'd have to have a Pope who's neutral on this new philosophy, but I see nothing besides religion being able to unify half of Italy in that fractured time. So with an agrarian movement uniting the south and demonizing the corrupt north of Italy, those states might seek to shelter in the OTL war-torn 18th Century by seeking admission to the HRE under Austrian domination. Venice would be problematic in this, but if Genoa, Milan, and Florence all join the HRE and the southern agrarian movement is powerful enough to make the Pope grant them safe passage across his states if they want to threaten the north.
Decades pass. It's an age of revolutions now: America, France, the first stirrings in Mexico and the Spanish Main, Russia turning more reactionary as Catherine the Great conquers the Tatar lands from the Ottomans.
A more progressive minded Austrian Kaiser wants to hold his Italian possessions against Napoleon's armies. He passes constitutional reform, but to deal with incipient nationalism that comes with the whole Rights of Man movement coming out of the Enlightenment, hits on a bright idea. Instead of conducting government business in German, the HRE will do its business in Latin, so that no subjects of the Constitutional Monarchy of the Holy Roman Empire are at a linguistic or cultural disadvantage.
This of course pisses off the Protestant north of Germany. A hundred and fifty years ago Europe rejected the politics of religion. They become increasingly anti-Vienna, maybe even begin to form a separate northern coalition without formally leaving the HRE. In the years ahead Prussia comes to dominate this Protestant League.
Italy is segmented into three states--the southern third of the HRE north of the Papal States, the Papal States as a kind of buffer state, and the radical aggrarian south which becomes increasingly radical, maybe even becomes a French ally as long as Boney agrees to reject the anti-clerical extremes of the Terror.
Then comes the Napleonic wars... Northern Italy depends on Austria and its eastern realms for the manpower to resist the French legions. The Protestant League and Russia join the coalition against Boney from the git-go, so he never gets a chance to redraw the borderlines of central Europe. But the burdens of war still manage to break the German union. With northern Italy more strongly defended, Napoleon's armies spend most of their time attacking their enemies north of Switerland, perhaps fighting to a draw in the Rhineland.
After the Peace of Copenhagen (ATL's Congress of Vienna, perhaps coming a few years earlier) Bonaparte hangs onto power in Paris, but loses his empire. The lowland countries become independent kingdoms a/o duchies. Poland remains triparted. And Germany splits along religious lines. with Bavaria and the Catholic upper Rhein valley joining the Eastern Empire (literally, Der Osterreich) along with Hungary, North Italy, Czecha, Slovakia, and northern Rumania becoming a Catholic Union.
But this center will not hold. Nationalism is coming. Increased literacy is coming. Latin is no substitute for a living language for public governance. Legislatures might speak a dead language, but local issues will demand the vulgar languages. More importantly, 1848 is coming. Constitutionalism is going to make the empowered classes turn conservative... and in the long run Italy will not be united with Austria.