I'll admit that a "Habsburg style" dual monarchy vias a vis Prussia and Poland has been a minor fascination, and not hardly impossible. Prussia, at least pre Napopeon, was by no means guaranteed to drive west towards Germany, this being mainly an after effect of Russias preponderance in Poland, and to an extent the Hababurgs still being a thing.
Whether such a thing could survive is another question- Prussia was Prussia in large part because it had to be, and because it had a lucky string of capable rulers- simply absorbing a big chunk of Poland would not have solved the latter's problems since I suspect such a kingdom would have many enemies both foreign and domestic and comparably few resources to spare, particularly if they dont get the Rhineland.
Well damn, you just gave me an idea.