@ Simon: the ruler of Austria would have liked to acquire Austria, but many of the other princes of the Empire, including Prussia, Saxony etc. weren't to keen on that prospect. Maybe if they had followed the advice of Eugene of Savoy, they could have make them accept it.
Except there is one problem with a territory swap, the Italian territories were the lands of Charles, not from Joseph; even ITTL since Charles would still be the Habsburg candidate in a War of the Spanish Succession, which resulted (and legitimated) those gains.
Regarding marriages, an interesting thought under this scenario might be if both brothers like IOTL only had daughters.
They might both want and decide to secure their own inheritance for their own daughter.
Also IOTL Maria Theresia was not elected as Queen Regnant, she was recognized as such by the Estates (there's a difference), at least in Hungary. Bohemia is more complex, but after the defeat of Charles VII, the Bohemian Estates didn't have much choice left. I remember reading that the disloyalty of the Bohemians, they initially preferred Charles VII, had left a negative impression on her.
However Maria Josepha (or depending on health; Maria Amalia) will also be recognized, at least internally. Foreign powers might see an opportunity they could seize...
@ Alpha Trion: like I wrote above, I can see both brothers wanting to be able to pass their inheritance on their own children; not to mention that if Charles ends up with Spain and the colonies, all other Powers will object against an union anyway. However if Charles ends up with Naples, Sicily (, Sardinia?) and Milan, then they might be interested in an union with Savoy (& Sardinia?) and that feeling would probably be mutual.
The position of Lorraine also depends on how peace treaties ITTL war of the Spanish Succession work out for them, they might have to accept that France gains their ancestral lands in exchange for the Southern Netherlands or Milan. They too would remain potential compromise candidates.